1869-Heat Considered as a Mode of Motion [Tyndall]

7/30/2019 1869-Heat Considered as a Mode of Motion [Tyndall]

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REESE LIBRARYOF THK

UNIVERSITY OF CALIFORNIA.Class


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HEATCONSIDERED AS

A MODE OF MOTION:BY

JOHN TYNDALL, F.R.S., &c.PBOFESSOR OF NATURAL PHILOSOPHY IN THE BOYAL INSTITUTIOK

AND IN THE EOYAL SCHOOL OF MINES.

FROM THE SECOND LONDON EDITION REVISED, WITH ADDITIONS EMBRACING TUBAUTHOIHa LATEST RESEARCHES.

rtp LI.S&,;

r . .^.

D. APPLETON & COMPANY,90, 92 & 94 GRAND STREET.1869.


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PEE FA E.IK the following Lectures I have endeavoured tobring the rudiments of a new philosophy within thereach of a person of ordinary intelligence and culture.The first seven Lectures of the course deal withthermometric heat ; its generation and consumption inmechanical processes ; the determination of the me-chanical equivalent of heat ; the conception of heat asmolecular motion ; the application of this conceptionto the solid, liquid, and gaseous forms of matter ; toexpansion and combustion ; to specific and latent heat ;and to calorific conduction.

The remaining five Lectures treat of radiant heat ;the interstellar medium, and the propagation of motionthrough this medium ; the relations of radiant heat toordinary matter in its several states of aggregation ;terrestrial, lunar, and solar radiation ; the constitutionof the sun ; the possible sources of his energy ; the re-lation of this energy to terrestrial forces, and to vege-table and animal life.My aim has been to rise to the level of these ques-tions from a basis so elementary, that a person possess-ing any imaginative faculty and power of concentration,might accompany me.

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IV PREFACE.Wherever additional remarks, or extracts, seemed

likelyto render the reader's knowledge of the subjectsreferred to in any Lecture more accurate or complete,

I have introduced such extracts, or remarks, as an Ap-pendix to the Lecture.For the use of the Plate at the end of the volume,I am indebted to the Council of the Eoyal Society ; itwas engraved to illustrate some of my own memoirs inthe i Philosophical Transactions.' For some of the"Woodcuts I am also indebted to the same learned body.To the scientific public, the names of the buildersof this new philosophy are already familiar. As ex-perimental contributors, Rumford, Davy, Faraday, andJoule, stand prominently forward. As theoretic writers(placing them alphabetically), we have Clausius, Helm-holtz, Kirchoff, Mayer, Eankine, Thomson ; and in thememoirs of these eminent men the student who desiresit, must seek a deeper acquaintance with the subject.MM. Regnault and Seguin also stand in honourable re-lationship to the Dynamical Theory of Heat, and M.Yerdet has recently published two lectures on it,marked by the learning for which he is conspicuous.To the English reader it is superfluous to mention thewell-known and highly-prized work of Mr. Grove.

I have called the philosophy of Heat a new philoso-phy, without, however, restricting the term to the sub-ject of Heat. The fact is, it cannot be so restricted ;for the connection of this agent with the general ener-gies of the universe is such, that if we master it per-fectly, we master all. Even now we can discern, thoughbut darkly, the greatness of the issues which connectthemselves with the progress we have made issueswhich were probably beyond the contemplation of


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PEEFACE. Vthose, by whose industry and genius the foundationsof our present knowledge were laid.

In a Lecture on the * Influence of the History ofScience on Intellectual Education,' delivered at theRoyal Institution, Dr. Whewell has shown { that everyadvance in intellectual education has been the effect ofsome considerable scientific discovery, or group of dis-coveries.' If the association here indicated be invari-able, then, assuredly, the views of the connection andinteraction of natural forces organic as well as inor-ganic vital as well as physical which have grown,and which are to grow, out of the investigation of thelaws and relations of Heat, will profoundly affect theintellectual discipline of the coming age.In the study of Nature two elements come intoplay, which belong respectively to the world of senseand to the world of thought. We observe a fact andseek to refer it to its laws, we apprehend the law, andseek to make it good in fact. The one is Theory, theother is Experiment ; which, when applied to the ordi-nary purposes of life, becomes Practical Science. Noth-ing could illustrate more forcibly the wholesome inter-action of these two elements, than the history of ourpresent subject. If the steam-engine had not been in-vented, we should assuredly stand below the theoreticlevel which we now occupy. The achievements ofHeat through the steam-engine have forced, with aug-mented emphasis, the question upon thinking minds4 "What is this agent, by means of which we can super-se'de the force of winds and rivers of horses and ofmen? Heat can produce mechanical force, and me-chanical force can produce Heat ; some common qualitymust therefore unite this agent and the ordinary forms


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VI PREFACE.of mechanical power.' This relationship established,the generalising intellect could pass at once to the otherenergies of the universe, and it now perceives the prin-ciple which unites them all. Thus the triumphs ofpractical skill have promoted the developement of phi-losophy. Thus, by the interaction of thought and fact,of truth conceived and truth executed, we have madeour science what it is, the noblest growth of moderntimes, though as yet but partially appealed to as asource of individual and national might.As a means of intellectual education its claims arestill disputed, though, once properly organised, greaterand more beneficent revolutions await its employmenthere, than those which have already marked its appli-cations in the material world. Surely the men whosenoble vocation it is to systernize the culture of England,can never allow this giant power to grow up in theirmidst without endeavouring to turn it to practical ac-count. Science does not need their protection, but itdesires their friendship on honourable terms : it wishesto work with them towards the great end of all educa-tion, the bettering of man's estate. By continuing todecline the offered hand, they invoke a contest whichcan have but one result. Science must grow. Its de-velopement is as necessary and as irresistible as themotion of the tides, or the flowing of the Gulf Stream.It is a phase of the energy of Nature, and as such issure, in due time, to compel the recognition, if not towin the alliance, of those who now decry its influenceand discourage its advance.

ROYAL INSTITUTION, February 1863.


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CONTENTS.LECTURE I.

Introduction Description of Instruments The Thermo-electric Pile and the Gal-vanometerHeat and Cold indicated by the Deflection of a Magnetic NeedleHeat generated by Friction, Compression, and Percussion "Waterfalls gen-

erate Heat Friction of Railway Axles The Force necessary to heat the Axlesis -withdrawn from the urging Force of the Engine Meteorites probably ren-dered incandescent by Friction against Air Rumford's Experiments on thoHeat excited by Friction Water boiled by Friction Consumption of Heatwhen compressed Air is suffered to expand Action of a Current of Air whenurged by a bellows against the Face of the Thermo-electric Pile . PAGE 13

APPENDIX TO LECTURE I.Mode of constructing a Thermo-electric Pile Mode of constructing a Galvanome-

ter Mode of rendering Needles astatic Experiments on the Magnetism ofGalvanometer Coils, and Mode of avoiding this Magnetism ... SO

LECTURE II.The Nature of Heat the Material Theory supposes it to be a Bubtle Fluid stored

up in the inter-atomic Spaces of Bodies The idea of 'Capacity' for Heatoriginated in this way The Dynamical Theory supposes Heat to be a Motionof the ultimate Particles of Bodies Rumford's and Davy's Views Davy'sFusion of Ice by Friction Bearing of the Experiment on the Material TheoryHeat and Light developed by the Compression of Air Ignition of Bisulphideof Carbon Vapour in Fire Syringe Thermal Effects of Air in Motion Con-densation of Aqueous Vapour by the Rarefaction of Air Machine of Schem-nitz Deportment of a Conductor between the Poles of a Magnet ApparentViscosity of the Magnetic Field The Conductor encounters Resistance to itsMotion A Conductor swiftly rotating is struck motionless when the Magnetis excited When tho Conductor is compelled to rotate Heat is generatedFusion of an Alloy by this Heat Measurement of the Amount of Heat gen-erated by a given Expenditure of Force Dr. Mayer and Mr. Joule The Me-chanical Equivalent of Heat Definition of the Term 'Foot-pound' Heatdeveloped increases as the height of the fall, and is proportional to the Squareof the Velocity Calculation of Heat generated by the impact of Projectiles


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Vlll CONTENTS.Heat equivalent to the Stoppage of the Earth in its Orbit Heat equivalent tothe Falling of the Earth into the Sun Preliminary Statement of MeteoricTheory of the Sun's Heat Analynis of Combustion Ignition of Diamond-Its Combustion in Oxygen due to the Showering of the Atoms of the Gasagainst the Surface of the Diamond Structure of Flame Candle and GasFlames Combustion on Mont Blanc The Light of Flames is materiallydiminished by the Rarefaction of the Air, though the Quantity of CombustibleMatter consumed remains the same Frankland's Experiments All Cases ofCombustion are duo to the Collision of Atoms which have been urged togetherby their mutual Attractions PAGE 37

APPENDIX TO LECTURE II.Extracts from the Twentieth Aphorism of the ' Novum Organum 'Abstract ofCount Rumford's Essay entitled ' An Enquiry concerning the Source of theHeat which is excited by Friction 'Note on the Compression of Bisulphideof Carbon Vapour f , . 67

LECTURE III.Expansion of Bodies by Heat Liberation of Particles from the thrall of CohesionThe Liquid and Gaseous States of Matter denned Illustrations of the Ex-

pansion of Air by Heat Ascent of Fire Balloon Gases expand by a constantIncrement for every Degree above 32 Fahr. Coefficient of Expansion Heat-ing of Gas at a constant Pressure Heating of Gas at a constant Volume Inthe former case Work is done by the Gas In the latter case no "Work is doneIn the former case an Excess of Heat equivalent to the Work done must be

imparted Calculation of the Mechanical Equivalent of Heat Mayer andJoule's Determinations Absolute Zero of Temperature Expansion withoutRefrigeration Expansion of Liquids Exceptional Deportment of Water andBismuth Energy of Atomic Forces Pyrometers Strains and Pressuressuperinduced by sudden Cooling Chilling of Metallic Wires by Stretching-Heating of India-rubber by Stretching Contraction of stretched India-rubberby Heat 74

APPENDIX TO LECTURE III.Further Remarks on Dilatation Linear, Superficial, and Cubic Coefficients of

Expansion The Thermometer Extracts from Sir Humphry Davy's FirstMemoir, entitled ' Heat, Light, and the Combinations of Light ' . , . 106

LECTURE IV.Vibrations and Tones produced by the contact of Bodies of different TemperaturesThe Trevelyan Instrument Rotation of hollow Spheres by Electricity-

Effect of Pressure on Fusing Point The Fusing Point of Bodies which con-tract on solidifying is raised by Pressure The Fusing Point of Bodies whichexpand on solidifying is lowered by Pressure Liquefaction of Ice by Pressure

Dissection of Ice by Calorific Beam Negative Crystallisation, Ice reducedinternally to Liquid Flowers, having six Petals each Central Spot a VacuumSound heard when the Spot is formed Physical Properties of Water fromwhich Air has been removed Its Cohesion enormously augmented It can be


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CONTENTS. IXheated far above its boiling Point Its Ebullition becomes Explosion Applica-tion of this Property to explain the Sound heard, when the central Spot isformed in Ice Possible bearing of this Property of Water on Boiler explosionsThe Boiling Point ofLiquids Resistance to Ebullition Cohesion of Particles,

adhesion to Vessel, external Pressure Boiling Points on various Alpine Sum-mits The law of Conservation illustrated in the Steam Engine The Geysersof Iceland Description of the Geysers and their Phenomena Bunsen's TheoryExperimental Illustration PAGB 114

APPENDIX TO LECTURE IV.Abstract of a Lecture on the Vibrations and Tones produced by the contact of

Bodies having different Temperatures Extracts from a Paper on the PhysicalProperties of Ice 144

LECTURE V.,Application of the Dynamical Theory to the Phenomena of Specific and LatentHeat Definition of Energy Potential and Djoiarnic Energy, illustrated by

the Raising and Falling of a Weight Convertibility of Potential into Dynam-ic Energy and the reverse Constancy of the Sum of both Energies Appli-- cation of the Ideas of Potential and Dynamic Energy to Atoms and MoleculesMagnitude of Molecular Forces The separation of a Body's Particles by Heat

is an Act the same in kind as the separation of a Weight from the EarthWork is here done within the heated Body Interior Work The Heat com-municated to a Body divides itself into Potential and Dynamic Energy AllSingle Atoms, whatever be their weight, possess the same amount of DynamicEnergy Specific Heat or Capacity for Heat explained by Reference to InteriorWork and to Atomic Number Experimental Illustrations of Specific Heat-Table of Specific Heats Influence of high Specific Heat of Water on ClimateHeat consumed in Change of Aggregation Latent Heat of Liquids and Va-pours It is Heat consumed in conferring Potential Energy on the ultimateParticles By Condensation and Liquefaction this Potential Energy is con-verted into Heat Mechanical Value of the Union of Oxygen and HydrogenMechanical Value of the Change from Steam to liquid Water MechanicalValue of the Change from liquid Water to solid Ice Experimental Illustra-tions Consumption and Generation of Heat by Changes of AggregationWater frozen by its own Evaporation The Cryophorus Solid Carbonic AcidThe Spheroidal State of Liquids In this State the Liquid is supported on aBed of its own Vapour Proofs that the Spheroidal Drop is not in Contact withthe hot Surface underneath it Experimental Illustrations of the SpheroidalCondition Possible Bearing of these Facts on the Fiery Ordeal, and on BoilerExplosions Freezing of Water and Mercury in red-hot Vessels . . 152

LECTURE VI.Convection in Air Larger physical Phenomena Winds caused by the heating

Action of the Sun The upper and lower Trade Winds Effect of the Earth'sRotation in changing the apparent Direction of Winds The Existence of thoupper Current proved by the Discharge of Ashes into it by Volcanoes Effectswhich accompany the apparent Motion of the Sun from side to side of theEquator Aqueous Vapour Tropical Rains Region of Calms Europe, forthe most Part in tho upper Trade Europe the Condenser of the Western At-1*


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X CONTENTS.lantic This is the Cause of the Mildness of European Temperature Rainfallin Ireland Effect of Mountain Ranges on Rainfall Convection in LiquidsExperimental Illustrations The Gulf Stream : its influence on the Climateof Britain Formation of Snow The Molecules aggregate to form FrozenStars with Rays sixty degrees apart Figures of Snow Crystals Collection ofSnow on Mountains The Snow Line Squeezing of this Snow to Ice Forma-tion of Glaciers The Motion of a Glacier resembles that of a River Theoriesof Glacier Motion The Regelation of Ice The Moulding of Ice by Pressure-Ancient Glaciers Their Traces in Switzerland, England, Ireland, and WalesThe Cedars of Lebanon grow on Glacier Moraines Theories of the GlacialEpoch Not due to a Diminution of Solar Power, or to the passage of the SolarSystem through cold Regions of Space ...... PAGE 185

APPENDIX TO LECTURE VI.Abstract of a Lecture on the Mer de Glaco 212

LECTURE VII.The Conduction of Heat a Transmission of Molecular Motion Different Bodies

possess different Powers of Transmission Good Conductors and bad Con-ductorsExperimental Illustrations Experiments of Ingenhausz, Despretz,Wiedemann, and Franz Table of Conductivities Parallelism of Conduction ofHeat and Conduction of Electricity Good Conductors of the one are also goodConductors of the other, and vice versa Influence of Heat on Electric Con-duction The Motion of Heat interferes with the Motion of Electricity Con-duction of Cold Constancy of Temperature of Animal Body Capacity tobear high Temperatures Diversion of Heat from the Purposes of Tempera-ture to the Performance of "Work Influence of Molecular Structure SomeBodies conduct differently in different Directions Conduction in Crystals andin Wood Feeble Conductivity of Organic Substances This secures themfrom sudden Alternations of Temperature Influence of Specific Heat on theSpeed of Conduction Anomalous Case of Bismuth as compared with Iron-Bismuth, though the worst Conductor, apparently transmits Heat mostspeedily Action of Clothing Rumford's Experiments Influence of mechani-cal Texture on Conduction A Powder conducts ill, on account of the incessantBreak in the Continuity of the Mass along which the Motion of Heat is trans-mitted Non-conductivity of Gypsum Effect of Boiler encrustations With-drawal of Heat from Flames by good Conductors The Motion of Flame,though intense, is much lowered by being transferred from BO light a Body toa heavy one Effect of Wire Gauze The Safety Lamp Conduction of Liquidsdenied by Rumford, but proved by M. Despretz Conduction of Gases deniedby Rumford, but affirmed in the case of Hydrogen by Prof. Magnus CoolingEffect of Air and Hydrogen Experiments on Gaseous Conduction doubt-ful 222

LECTURE VIII.Radiant Heat Cooling a loss of Motion To what is this Motion imparted? Pre-

liminary Experiments on Sound Communication of Vibrations through theAir to Membranes and to Flames The Vibrations of a Body propagatedthrough the Air and striking on the Drum of the Ear produce Sound Analo-gous Phenomena of Light Theories of Emission and Undulation Discussion*


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CONTENTS. XIon tho Subject Newton Huyghens Euler Young Frcsnel Space filledwith an elastic Medium called Ether The Motion of a hot or of a luminousbody communicated to this Ether is propagated through it in waves In thisform Heat is called Radiant HeatThQ Thermo-electric Pile in relation toRadiant Heat Distribution of Heat in the Electric Spectrum examined ex-perimentallyLow Calorific Power of blue End of Spectrum The mostluminous Part not the hottest Part Of the visible rays Red is the hottestThe maximum Calorific Action is beyond the Red, and is due to Rays which areincompetent to excite the Sense of Vision Extra Violet Rays Physical Causeof Colour The Spectrum is to the Eye what the Gamut is to the Ear TheColour of Light corresponds to the Pitch of Sound Number of Impulses in-volved in the Perception of Light Theory of Exchanges Reflection of RadiantHeat from Plane Surfaces Angle of Incidence equal to the Anglo of Reflec-tion Experimental Proof The obscure Rays of the Electric Lamp pursue thesame Track as the luminous ones Angular Velocity of reflected Ray twicethat of rotating Mirror Experiments with radiant Heat of Fire and of obscureBodies Reflection from curved Surfaces Parabolic Mirrors Explosion ofChlorine and Hydrogen in Focus of Mirror by Light Explosion of Oxygenand Hydrogen by Radiant Heat Reflection of Cold . . . PAGE 261

APPENDIX TO LECTURE VIII.On the Sounds produced by the Combustion of Gases in Tubes . . . 288

LECTURE IX.The Intensity of Radiant Heat diminishes as the Square of the Distance from tho

radiant Point increases Experimental Proof .Undulations of Sound longitu-dinal, of Light transversal The ultimate Particles of different Bodies possessdifferent Powers of Communicating Motion to the Ether Experimental Illus-trations of good and bad Radiators Reciprocity of Radiation and Absorption

Protection by Gilding against Radiant Heat Transmission of Radiant Heatthrough Solids and Liquids Diathermancy Absorption occurs within thoBody Absorption of Light by Water Radiant Heat passes through Diather-mic Bodies without heating them Athermic Bodies are heated Concentra-tion of Beam on bulb of Air Thermometer Penetrative Power of Sunbeams

Sifting of Radiant Heat Ratio of Obscure to Luminous Radiation in variousFlames 301APPENDIX TO LECTURE IX.

On some physical Properties of Ice 328LECTURE X.

Absorption of Heat by Gases and Vapours First Apparatus Rockealt Plates-Peculiarities of the Galvanometer The higher Degrees of greater Value thanthe lower ones Improved Apparatus Principle of Compensation permits theuse of a powerful Source of Heat while it preserves the Needle in a sensitivePosition Air, Oxygen, Hydrogen, and Nitrogen are practical Vacua to RadiantHeat Opacity of Olofiant Gas and Sulphuric Ether Radiation through otherGases and Vapours Great difference of Absorbing Power Radiation of Heatby Gases The atom which Absorbs powerfully Radiates powerfully Absorp-tion by Gases at a Tension of an Atmosphere Absorptions at smaller Tensions


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Xll CONTENTS.Comparison of Elementary and Compound Gases and Vapours Radiation

through Lampblack, &c PAGE 341APPENDIX TO LECTURE X.

Calibration of the Galvanometer 3T6LECTURE XL

Action of Odorous Substances on Radiant Heat List of Perfumes examined Ac-tion of Ozone on Radiant Heat Influence of the Size of the Electrodes on theQuantity of Ozone generated Constitution of Ozone Radiation and Absorp-tion of Gases and Vapours determined without any Source of Heat external tothe gaseous Body itself Dynamic Radiation and Absorption Varnishing ametal Surface by a Gas Varnishing of a Gas by a Vapour Tenuity of Bora-cic Ether shown in Experiments on Dynamic Radiation Influence of Lengthof radiating Column In a long Tube, a Vapour at a small Tension may ex-ceed a Gas at a high Tension, while in a short Tube the Gas exceeds theVapour Radiation through Humid Air Action of the Vapour of the Atmo-sphere on Terrestial and Solar Radiation Objections answered Applicabilityof Rocksalt Plates Experiments in Tube without Plates Experiments with-out either Tube or Plates Examination of Air from various localities Influ-ence of the Results on the Science of Meteorology Application to tropicalRain Torrents To the Formation of Cumuli To the Condensation of a Moun-tainous Region To Radiation Experiments at high and low Elevations Tothe Cold of Central Asia To the Thermometric Range in Australia To theMeteorology of Sahara To Leslie's Experiments To "VVells's Theory of Ice-formation in India To Melloni's Theory of Serein , 379

APPENDIX TO LECTUKE XI.Extracts from a Discourse on Eadiation through the Earth's Atmosphere Thermo-

metric range in Asia, Africa, and Australia . 415

LECTURE XII.Examination of the Diathermancy of Volatile Liquids and their Vapours Apparatus

for this purpose Eocksalt Cell Platinum Lamp Experimental arrangement ofApparatus for determining Absorption of Heat by Liquids Table of the Absorp-tion of Heat by Liquids at various thicknesses Table of the Absorption of Heatof the same quality by the Vapours of those Liquids Absorption of Heat by thsame Vapours when the quantities of Vapour are proportional to the quantitiesof Liquid Comparative View of the Action of Liquids and their Vapours onRadiant Heat Predominance of Liquid Water as an Absorbent fixes the predom-inance of its Vapour Physical cause of Transparency and Opacity Transpar-ency defined as the Discord, and Opacity as the Accord, between the VibratingPeriods of the Source, and the interposed Substance Influence of Temperatureon the Transmission of Eadiant Heat Changes of Position through changes ofTemperature Eadiation from various Flames through Vapours Periods of Vibration of a Hydrogen Flame proved to coincide with those of cool Aqueous Vapour The same true of a Carbonic Oxide Flame and cool Carbonic Acid Physi-cal Analysis of the Human Breath Eadiation through Liquids, with a Hydrogen


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CONTENTS. X1HFlame and a Platinum Spiral in a Hydrogen Flame as Sources of Heat Exptona-"tion of certain results obtained by Melloni and Knoblauch . . PAGE 423

APPENDIX TO LECTURE XII.On Luminous and Obscure Eadiation 456

LECTURE XIII.Dew : a clear Sky and calm but deep Atmosphere necessary for its copious Forma-

tion Dewed Substances colder than undewed ones Dewed Substances betterEadiators than undewed ones Theory of Wells Dew is the Condensation of theAtmospheric Vapour on Substances which have been chilled by Eadiation LunarEadiation Constitution of the Sun The bright Lines in the Spectra of the Met-als An incandescent Vapour absorbs the Eays which it can itself emit Kirch-hoff's Generalisation Fraunhofer's Lines, caused by the Absorption of such Eaysby the luminous Solar Atmosphere as that Atmosphere itself could emit SolarChemistry Emission by the Sun Herschel and Pouillet's Experiments Mayer'sMeteoric Theory Eflect of the Tides on the Earth's Eotation Energies of theSolar System Ilelmholtz, Herschel, Thomson, Waterston Eelation of the Sunto Vegetable and Animal Life Form of Solar Energy in Plants and Animals 470

APPENDIX TO LECTURE XIII.Extract from a Lecture on the Physical Basis of Solar Chemistry Extract from a

Paper by Dr. Joule Extracts from Dr. Mayer's Paper on Organic Motion andNutrition . 516


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HEATCONSIDERED AS

A MODE OF MOTION.LECTURE I.[January 23, 1862.]

INSTRUMENTS GENERATION OF HEAT BY MECHANICAL ACTIONCONSUMPTION OP HEAT IN WORK.

APPENDIX : NOTES ON THE THERMO-ELECTRIC PILE AND GALVANOMETER.

THEaspects of nature provoke in man the spirit of

enquiry. As the eye is made for seeing, and theear for hearing, so the human mind is formed for under-standing the phenomena of the material universe. Thenatural philosophy of our day results from the irrepressi-ble exercise of this endowment. One great characteristicof Natural Science is its growth ; all its facts are fruitful,every new discovery becoming instantly the germ of freshinvestigation. But no nobler example of this growthcould be adduced than the expansion and developmentwhich men's thoughts and knowledge have undergonewithin the last two-and-twenty years, with reference to thesubject which is now to occupy our attention. In scien-tific manuals, only scanty reference has, as yet, beenmade to the modern philosophy of Heat, and thus thepublic knowledge regarding it remains below the attain-


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14 LECTUKE I.able level. But the reserve is natural, for the subject isstill an entangled one, and, in entering upon it, we must beprepared to encounter difficulties. In the whole range ofNatural Science, however, there are none more worthy ofbeing overcome, none whose subjugation secures a greaterreward to the worker. For by mastering the laws and re-lations of Heat, we make clear to our minds the interde-pendence of natural forces generally. Let us, then, com-mence our labours with heart and hope ; let us familiariseourselves with the latest facts and conceptions regardingthis all-pervading agent, and seek diligently the links oflaw which underlie the facts and give unity to their mostdiverse appearances. If we succeed here we shall satisfy,to an extent unknown before, that love of order and ofbeauty which, I am persuaded, is implanted in the mindof every person here present. From the heights at whichwe aim, we shall have nobler glimpses of the system ofNature than could possibly be obtained, if I, while actingas your guide in the region which we are now about to en-ter, were to confine myself to its lower levels and alreadytrodden roads.

It is my first duty to make you acquainted with someof the instruments which I intend to employ in the exami-nation of this question. I must devise some means ofmaking the indications of heat and cold visible to you all,and for this purpose an ordinary thermometer would beuseless. You could not see its action ; and I am anxiousthat you should see, with your own eyes, the facts onwhich our subsequent philosophy is to be based. I wish togive you the material on which an independent judgmentmay be founded ; to enable you to reason as I reason if youdeem me right, to correct me if I go astray, and to censureme if you find me dealing unfairly with my subject. Tosecure these ends, I have been obliged to abandon the useof a common thermometer, and to resort to the little in-


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THE THEKMO-ELECTEIC PILE AND GALVANOMETER. 15strument A B (fig. 1), which you see before me on the table,and which is called a thermo-electric pile.*

By means of this instrument I cause the heat which itreceives to generate an electric current. You know, orought to knoAV, that such a current has the power of de-flecting a freely suspended magnetic needle, to which itflows parallel. Before you I have placed such a needlem n (fig. 1), surrounded by a covered copper wire, the free

Fig. 1.

ends of which, w w are connected with the thermo-electricpile. The needle is suspended by a fibre, s s, of unspunsilk, and protected by a glass shade, G, from any disturb-ance by currents of air. To one end of the needle I havefixed a piece of red paper, and to the other end a piece ofblue. All of you see these pieces of paper, and when theneedle moves, its motion will be clearly visible to the mostdistant person in this room.f

* A brief description of the thermo-electric pile is given in theAppendix to this Lecture.

f In the actual arrangement the galvanometer here described stood on.a stool in front of the lecture table, the wires w w, being sufficiently long


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16 LECTURE I.At the present moment the needle is quite at rest, and

pointsto the zero mark on the graduated disc underneath

it. This shows that there is no current passing. ' I nowbreathe for an instant against the naked face A of the pile

a single puff of breath is sufficient for my purposeobserve the effect. The needle starts off and passes throughan arc of 90. It would go further, did I not limit itsswing by fixing, edgewise, a thin plate of mica at 90.Take notice of the direction of the deflection ; the red endof the needle moved from me towards you, as if it dislikedme, and had been inspired by a sudden affection for you.This action of the needle is produced by the small amountof warmth communicated by my breath to the face of thepile, and no ordinary thermometer could give so large andprompt an indication. We will let the heat thus communi-cated waste itself; it will do so in a very short time, andyou notice, as the pile cools, that the needle returns to itsfirst position. Observe, now, the effect of cold on the faceof the pile. I have here some ice, but I do not wish to wetmy instrument by touching it with ice. Instead of doingso, I will cool this plate of metal by placing it on the ice ;then wipe the chilled metal, and touch with it the face ofthe pile. You see the effect ; a moment's contact sufficesto produce a prompt and energetic deflection of the needle.But mark the direction of the deflection. When the pilewas warmed, the red end of the needle moved from metowards you ; now its likings are reversed, and the red endmoves from you towards me. Thus you see that cold andheat cause the needle to move in opposite directions. Theimportant point here established is, that from the directionin which the needle moves, we can, with certainty, inferwhether cold or heat has been communicated to the pile ;to reach from the table to the stool ; for a further description of the gal-vanometer, see the Appendix to this Lecture.


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HEAT OF FRICTION. ITand the energy with which the needle moves the prompt-ness with which it is driven aside from its position of rest

gives us some idea of the comparative quantity of heator cold imparted to it in different cases. In a future lectureI shall explain how we may express the relative quantitiesof heat with numerical accuracy ; but for the present a gen-eral knowledge of the action of our instruments will besufficient.

My desire now is to connect heat with the more famil-iar forms of force, and I will, therefore, in the first place,try to furnish you with a store of facts illustrative of .thegeneration of heat by mechanical processes. I have placedsome pieces of wood in the next room, which my assistantwill now hand to me. Why have I placed them there ?Simply that I may perform my experiments with that sin-cerity of mind and act which science demands from hercultivators. I know that the temperature of that room isslightly lower than the temperature of this one, and thathence the wood which is now before me must be slightlycolder than the face of the pile with which I intend to testthe temperature of the wood. Let us prove this. I placethe face of the pile against this piece of wood ; the red endof the needle moves from you towards me, thus showingthat the contact has chilled the pile. I now carefully rubthe face of the pile along the surface of the wood ; I say' carefully,' because the pile is a brittle instrument, andrough usage would destroy it ; mark what occurs. Theprompt and energetic motion of the needle towards youdeclares that the face of the pile has been heated by thissmall amount of friction. The needle, you observe, goesquite up to 90 on the side opposite to that towards whichit moved before the friction was applied.Now these experiments, which illustrate the develope-ment of heat by mechanical means, must be to us what aboy's school exercises are to him. In order to fix them on


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18 LECTUKE I.our minds, and obtain due mastery over them, we must re-peat and vary them in many ways. In this task I ask youto accompany me. Here is a flat piece of brass with a stemattached to it ; I take the stem in my fingers, preservingthe brass from all contact with my warm hand, by envelop-ing the stem in cold flannel. I place the brass in contactwith the face of my pile ; the needle moves, showing thatthe brass is cold. I now rub the brass against the surfaceof this cold piece of wood, and lay it once more againstmy pile. I withdraw it instantly, for it is so hot that if Iallowed it to remain in contact with the instrument, thecurrent generated would dash my needle violently againstits stops, and probably derange its magnetism. You seethe strong deflection which even an instant's contact canproduce. Indeed, when a boy at school, I have often blis-tered my hand by the contact of a brass button, which Ihad rubbed energetically against a form. Here, also, is arazor, cooled by contact with ice ; and here is a hone, with-out oil, along which I rub my cool razor, as if to sharpenit. I now place the razor against the face of the pile,and you see that the steel, which a minute ago was cold,is now hot. Similarly, I take this knife and knife-board,which are both cold, and rub the knife along the board. Iplace the knife against the pile, and you observe the result ;a powerful deflection, which declares the knife to be hot.I pass this cold saw through this cold piece of wood, andplace, in the first instance, the surface of the wood againstwhich the saw has rubbed, in contact with the pile. Theneedle instantly moves in a direction which shows thewood to be heated. I allow the needle to return to zero,and now apply the saw to the pile. It also is hot. Theseare the simplest and most common-place examples of thegeneration of heat by friction, and I choose them for thisreason. Mean as they appear, they will lead us by degrees


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HEAT OF COMPRESSION AND PERCUSSION. 19into the secret recesses of Nature, and lay open to ourview the policy of the material universe.Let me now make an experiment to illustrate the de-velopement of heat by compression. I have here a pieceof deal, cooled below the temperature of the room, andgiving, when placed in contact with our pile, the deflectionwhich indicates cold. I place this wood between theplates of a small hydraulic press, and squeeze it forcibly.The plates of the press are also, you will observe, coolerthan the air of the room. After compression, I bring thewood into contact with the pile ; see the effect. The gal-vanometer declares that heat has been developed by theact of compression. Precisely the same occurs when Iplace this lead bullet between the plates of the press andsqueeze it thus to flatness.And now for the effect of percussion. I have here acold lead bullet, which I place upon this cold anvil, andstrike it with a cold sledge hammer. The sledge descendswith a certain mechanical force, and its motion is suddenlydestroyed by the bullet and anvil ; apparently the force ofthe sledge is lost. But let us examine the lead ; you see itis heated, and could we gather up all the heat generated bythe shock of the sledge, and apply it without loss mechan-ically, we should be able, by means of it, to lift this ham-mer to the height from which it fell.

I have here arranged another experiment, which is almosttoo delicate to be performed by the coarse apparatus neces-sary in a lecture, but which I have made several times be-fore entering this room to-day. Into this small basin Ipour a quantity of mercury which has been cooled in thenext room. I have coated one of the faces of my thermo-.electric pile with varnish, so as to defend it from the mer-cury, which would otherwise destroy the pile ; and, thusprotected, I can, as you observe, plunge the pile into theliquid metal. The deflection of the needle shows you that


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20 LECTURE I.

Fig. 2.

the mercury is cold. Here are two glasses A and B (fig.2),

swathed thickly round by listing, which will effectuallyprevent the warmth of my hands from reaching the mer-cury. "Well, I pour the cold mercury from the one glassinto the other, and back. It falls with a certain mechani-cal force, its motion is destroyed, but heat is developed.The amount of heat generated by a single pouring out isextremely small ; I could tell you the exact amount, butshall defer quantitative considerations till our next lecture ;so I pour the mercury from glass to glass ten or fifteentimes. Now mark the result, when the pile is plunged into

the mercury. Theneedle moves, and itsmotion declares thatthe mercury, whichat the beginning ofthe experiment wascooler than the pile,is now warmer thanthe pile. We hereintroduce into thelecture-room an effectwhich occurs in na-ture at the base of ev-ery waterfall. Thereare friends before me

who have stood amid the foam of Niagara. Had they,when there, dipped sufficiently sensitive thermometers intothe water at the top and bottom of the cataract, they wouldhave found the latter a little warmer than the former. Thesailor's tradition, also, is theoretically correct ; the sea isrendered warmer through the agitation produced by astorm, the mechanical dash of its billows being ultimatelyconverted into heat.

Whenever friction is overcome, heat is produced, and


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ETC., FLlfrT AOT) STEEL. 21the heat produced is the measure of the force expended inovercoming the friction. The heat is simply the primitiveforce in another form, and if we wish to avoid this conver-sion, we must abolish the friction. We usually put oilupon the surface of a hone, we grease a saw, and are care-ful to lubricate the axles of our railway carriages. Whatare we really doing in these cases ? Let us get generalnotions first ; we shall come to particulars afterwards. It isthe object of a railway engineer to urge his train bodilyfrom one place to another ; say from London to Edinburgh,or from London to Oxford, as the case may be ; he wishesto apply the force of his steam, or of his furnace, whichgives tension to the steam, to this particular purpose.It is not his interest to allow any portion of that force tobe converted into another form of force which would notfurther the attainment of his object. He does not wanthis axles heated, and hence he avoids as much as possibleexpending his power in heating them. In fact, he has ob-tained his force from heat, and it is not his object to recon-vert the force thus obtained into its primitive form. For,for every degree of temperature generated by the frictionof his axles, a definite amount would be withdrawn fromthe urging force of his engine. There is no force lost ab-solutely. Could we gather up all the heat generated bythe friction, and could we apply it all mechanically, vreshould, by it, be able to impart to the train the preciseamount of speed which it had lost by the friction. Thusevery one of those railway porters whom you see movingabout with his can of yellow grease, and opening the littleboxes which surround the carriage axles, is, without know-ing it, illustrating a principle which forms the very solderof Nature. In so doing, he is unconsciously affirming boththe convertibility and the indestructibility of force. He ispractically asserting that mechanical energy may be con-verted into heat, and that, when so converted, it cannot


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22 LECTURE I.still exist as mechanical energy, but that, for every degreeof heat developed, a strict and proportional equivalent ofthe locomotive force of the engine disappears. A station isapproached, say at the rate of thirty or forty miles an hour ;the brake is applied, and smoke and sparks issue from thewheel on which it presses. The train is brought to restHow ? Simply by converting the entire moving force whichit possessed, at the moment the brake was applied, into heat.

So, also, with regard to the greasing of a saw by a car-penter. He applies the muscular force of his arm with theexpress object of getting through the wood. He wishes totear the wood asunder, to overcome its mechanical cohesionby the teeth of his saw. When the saw moves stiffly, onaccount of the friction against its flat surface, the sameamount of force may produce a much smaller effect thanwhen the implement moves without friction. But in whatsense smaller ? Not absolutely so, but smaller as regardsthe act of sawing. The force not expended in the sawingis not lost ; it is converted into heat, and I gave you anexample of this a few minutes ago. Here again, if wecould collect the heat engendered by the friction, and applyit to urge the saw, we should make good the precise amountof work which the carpenter, by neglecting the lubricationof his implement, had simply converted into another formof power.We warm our hands by rubbing, and in the case offrostbite we thus restore the

necessary heat to the injuredparts. Savages have the art of producing fire by the skil-ful friction of well-chosen pieces of wood. It is easy tochar wood in a lathe by friction. From the feet of thelabourers on the roads of Hampshire sparks issue copiouslyon a dark night, the collision of their iron-shod shoesagainst the flints producing the effect. In the commonflint and steel the particles of the metal struck off are somuch heated by the collision that they take fire and burn in


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WATEK BOILED BY FRICTION. 25temperature as 178. At the end of two hours and twentyminutes it was 200, and at two hours and thirty minutesfrom the commencement the water actually boiled! Rum-ford's description of the effect of this experiment on thosewho witnessed it, is quite delightful. 4 It would be diffi-cult,' he says, 4 to describe the surprise and astonishmentexpressed in the countenances of the bystanders on seeingso large a quantity of water heated, and actually made toboil, without any fire. Though there was nothing thatcould be considered very surprising in this matter, yet Iacknowledge fairly that it afforded me a degree of childishpleasure which, were I ambitious of the reputation of agrave philosopher, I ought most certainly rather to hidethan to discover.'* I am sure that both you and I can dis-pense with the application of any philosophy which wouldstifle such emotion as Rumford here avowed. In connec-tion with this striking experiment, Mr. Joulef has estimatedthe amount of mechanical force expended in producing theheat, and obtained a result which ' is not very widely differ-ent ' from that which greater knowledge and more refinedexperiments enabled Mr. Joule himself to obtain, as regardsthe numerical equivalence of heat and work.

It would be absurd on my part to attempt here a repe-tition of the experiment of Count Rumford with all itsconditions. I cannot devote two hours and a half to a sin-gle experiment, but I hope to be able to show you substan-tially the same effect in two minutes and a half. I havehere a brass tube, four inches long, and three quarters ofan inch in interior diameter. It is stopped at the bottom,and I thus screw it on to a whirling table, by means ofwhich I can cause the upright tube to rotate very rapidly.I have here two pieces of oak wood, united by a hinge, and

2

* Rumford's Essays, vol. ii. p. 484.f Philosophical Transactions, vol. cxl. p. 62.


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26 LECTURE I.in which are two semicircular grooves, which are intendedto embrace the brass tube. Thus the pieces of wood forma kind of tongs, T (fig. 3), by gently squeezing which I canproduce friction between the wood and the brass tube,when the latter rotates. I almost fill the tube with cold

Fig. 3.

water, and stop it with a cork, to prevent the splashing outof the liquid, and now I put the machine in motion. Asthe action continues, the temperature of the water rises,and though the two minutes and a half have not yet elapsed,those near the apparatus will see steam escaping from thecork. Three or four times to-day I have projected the corkby the force of the steam to a height of twenty feet in theair. There it goes again, and the steam follows it, pro-ducing by its precipitation this small cloud in the at-mosphere.

In all the cases hitherto introduced to your notice, heathas been generated by the expenditure of mechanical force.Our experiments have gone to show that where mechanicalforce is expended heat is produced, and I wish now tobring before you the converse experiment, that is, the con-sumption of heat in mechanical work. And should you atpresent find it difficult to form distinct conceptions as tothe bearing of these experiments, I exhort you to be pa-tient. We are engaged on a difficult and entangled sub-


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COLD OF DILATATION. If " 27ject, which, I hope, we shall disentangle aa we go along.I have here a strong vessel, filled, at the present moment,with compressed air. It has been now compressed for somehours, so that the temperature of the air within the vesselis the same as that of the air of the room without it. Atthe present moment, then, this inner air is pressing againstthe sides of the vessel, and if I open this cock a portion ofthe air will rush violently out of the vessel. The word4 rush,' however, but vaguely expresses the true state ofthings ; the air which rushes out is driven out by the airbehind it ; this latter accomplishes the work of urging for-ward the stream of air. And what will be the conditionof the working air during this process ? It will be chilled.It performs mechanical wrork, and the only agent which itcan call upon to perform it is the heat which it possesses,and to which the elastic force with which it presses againstthe sides of the vessel, is entirely due. A portion of thisheat will be consumed and the air will be chilled. Observethe experiment which I am about to make. I will turnthe cock c, and allow the current of air from the vessel v(fig. 4), to strike against the face of the pile P. See how

Fig. 4.


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28 LECTTTEE I.the magnetic needle responds to the act ; its red end isdriven towards me, thus declaring that the pile has beenchilled by the current of air.The effect is different when a current of air is urgedfrom the nozzle of a common bellows against the thermo-electric pile. In the last experiment the mechanical workof urging the air forward was performed by the air itself,and a portion of its heat was consumed in the effort. Inthe case of the bellows, it is my muscles which performthe work. I raise the upper board of the bellows and theair rushes in ; I press the boards with a certain force, andthe air rushes out. The expelled air strikes the face of thepile, has its motion stopped, and an amount of heat equiva-lent to the destruction of this motion is instantly generated.Thus you observe that when I urge with the bellows B

Fig. 5.

(fig. 5), a current of air against the pile, the red end of theneedle moves towards you, thereby showing that the faceof the pile has been, in this instance, warmed by the air. Ihave here a bottle of soda water ; at present the bottle isslightly warmer than the pile, as you see by the deflectionit produces ; I cut the strings which holds the cork, and it


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COLD OF DILATATION. 29is it driven out by the elastic force of the carbonic acidgas ; the gas performs work, in so doing consumes heat,and now the deflection it produces is that of cold. Thetruest romance is to be found in the details of daily life,and here, in operations with which every child is familiar,we shall gradually discern the illustration of principlesfrom which all material phenomena flow.


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APPENDIX TO LECTURE I.

Fig. 6.

NOTE ON THE CONSTEUCTION OF THE THEEMO-ELECTEIC PILE.LET A B (fig. 6) be a bar of antimony, and B c a bar of bis-

muth, and let both bars be soldered together at B. Let the freeends A and c be united by a piece of wire, ADC.On warming the place of junction, B, an electriccurrent is generated, the direction of which isfrom bismuth to antimony (B to A, or againstthe alphabet), across the junction, and from an-timony to bismuth (A to B, or with the alpha-bet), through the connecting wire, ADC. Thearrow indicates the direction of the current.

If the junction B be chilled, a current is gene-rated opposed in direction to the former. Thefigure represents what is called a thermo-electricpair or couple.

By the union of several thermo-electric pairs,a more powerful current can be generated thanwould be obtained from a single pair. Fig. 7,for example, represents such an arrangement, in which the shadedbars are supposed to be all of bismuth, and the unshaded ones ofantimony ; on warming all the junctions, B, B, &c., a current isgenerated in each, and the sum of these currents, all of which flowin the same direction, will produce a stronger resultant currentthan that obtained from a single pair.The V formed by each pair need not be so wide as it is shownin fig. 7 ; it may be contracted without prejudice to the couple.And if it is desired to pack several pairs into a small compass,


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THEKMO-ELECTEICITY. 31each separate couple may be arranged as in fig. 8, where the blacklines represent small bismuth bars, and the white ones small barsof antimony. They are soldered together at the ends, and through-out the length are usually separated by strips of paper merely. A

collection of pairs thus compactly set together constitutes a ther-mo-electric pile, a drawing of which is given in fig. 9.The current produced by heat being always from bismuth toantimony across the heated junction, a moment's inspection offig. 7 will show that when any one of the junctions A, A, is heated,a current is generated, opposed in direction to that generatedwhen the heat is applied to the junctions B, B. Hence, in the caseof the thermo-electric pile, the effect of heat falling upon its two

Fig. 8. Fig. 9.

opposite faces is to produce currents in opposite directions. Ifthe temperature of the two faces be alike, they neutralize eachother, no matter how high they may be heated absolutely, but ifone of them be warmer than the other, a current is produced. Thecurrent is thus due to a difference of temperature between the twofaces of the pile, and within certain limits the strength of the cur-rent is exactly proportioned to this difference.


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32 APPENDIX TO LECTUKE I.From the junction of almost any other two metals, thermo-

electric currents may be obtained, but they are most copiouslygenerated by the union of bismuth and antimony.*

NOTE ON THE CONSTRUCTION OF THE GALVANOMETER.The existence and direction of an electric current are shown

by its action upon a freely suspended magnetic needle.But such a needle is held in the magnetic meridian by themagnetic force of the earth. Hence, to move a single needle, thecurrent must overcome the magnetic force of the earth.

Very feeble currents are incompetent to do this in a sufficientlysensible degree. The following two expedients are, therefore,combined to render sensible the action of such feeble currents :

The wire through which the current flows is coiled so as tosurround the needle several times ; the needle must swing freelywithin the coil. The action of the single current is thus mul-tiplied.The second device is to neutralize the directive force of theearth, without prejudice to the magnetism of the needle. This isaccomplished by using two nee-dles instead of one, attachingthem to a common vertical stem,and bringing their opposite polesover each other, the north end ofthe one needle, and the south end .of the other, being thus turned inthe same direction. The doubleneedle is represented in fig. 10. 2It must be so arranged thatone of the needles shall be within the coil through which the cur-

* The discovery of thermo-electricity is due to Thomas Seebeck, Pro-fessor in the University of Berlin. Nobili constructed the first thermo-electric pile ; but in Melloni's hands it became an instrument so importantas to supersede all others in researches on radiant heat. To this purposeit will be applied in future lectures.


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THE ASTATIC NEEDLE. 33rent flows, while the other needle swings freely above the coil, thevertical connecting piece passing through an appropriate slit inthe coil. Were both the needles within, the same current wouldurge them in opposite directions, and thus one needle would neu-tralize the other. But when one is within and the other without,the current urges both needles in the same direction.The way to prepare such a pair of needles is this. Magnetizeboth of them to saturation ; then suspend them in a vessel, or un-der a shade, so as to protect them from air-currents. The systemwill probably set in the magnetic meridian, one needle being inalmost all cases stronger than the other; weaken the strongerneedle carefully by the touch of a second smaller magnet. Whenthe needles are precisely equal in strength, they will set at rightangles to the magnetic meridian.

It might be supposed that when the needles are equal instrength, the directive force of the earth would be completely an-nulled, that the double needle would be perfectly astatic, and per-

Fig. 11.

fectly neutral as regards direction ; obeying simply the torsion ofits suspending fibre. This would be the case if the magnetic axesof both needles could be caused to lie with mathematical accuracyin the same vertical plane. In practice, this is next to impos-2*


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34: APPENDIX TO LECTURE I.sible ; tlie axes always cross each other. Let n s, n' s' (fig. 11)represent the axes of two needles thus crossing, the magneticmeridian being parallel to M E ; let th. pole n be drawn by theearth's attractive force in the direction n m ; the pole s' beingurged by the repulsion of the earth in a precisely opposite direc-

' tion. When the poles n and s' are of exactly equal strength, it ismanifest that the force acting on the pole s', in the case here sup-posed, would have the advantage as regards leverage, and wouldtherefore overcome the force acting on n. The crossed needleswould therefore turn away still further from the magnetic meri-dian, and a little reflection will show that they cannot come to restuntil the line which bisects the angle enclosed by the needles is atright angles to the magnetic meridian.

This is the test of perfect equality as regards thv, magnetismof the needles ; but in bringing the needles to this state of perfec-tion, we have often to pass through various stages of obliquity tothe magnetic meridian. In these cases the superior strength ofone needle is compensated by an advantage, as regards leverage,possessed by the other. By a happy accident a touch is some-times sufficient to make the needles perfectly equal ; but manyhours are often expended in securing this result. It is only, ofcourse, in very delicate experiments that this perfect equality isneeded ; but in such experiments it is essential.

Another grave difficulty has beset experimenters, even after theperfect magnetization of their needles has been accomplished.Such needles are sensitive to the slightest magnetic action, andthe covered copper wire, of which the galvanometer coils areformed, usually contains a trace of iron sufficient to deflect theprepared needle from its true position. I have had coils in whichthis deflection amounted to 30 degrees ; and in the splendid in-struments used by Professor Du Bois Raymond, in his researcheson animal electricity, the deflection by the coil is sometimes evengreater than this. Melloni encountered this difficulty, andproposed that the wires should be drawn through agate holes,thus avoiding all contact with iron or steel. The disturb-ance has always been ascribed to a trace of iron containedin the copper wire. Pure silver has also been proposed insteadof copper.


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THE ASTATIC NEEDLE. 35To pursue his beautiful thermo-electric researches in a satisfac-

tory manner, Professor Magnus, of Berlin, obtained pure copper,by a most laborious electrolytic process, and after the metal hadbeen obtained, it required to be melted eight times in successionbefore it could be drawn inio wire. In fact, the impurity of thecoil entirely vitiated the accuracy of the instrument, and almostany amount of labour would be well expended in removing thisgreat defect.My own experience of this subject is instructive. I had a beau-tiful instrument constructed a few years ago by Sauerwald, of Ber-lin, the coil of which, when no current flowed through it, deflectedmy double needle full 30 degrees from the zero line. It was im-possible to attain quantitative accuracy with this instrument.

I had the wire removed by Mr. Becker, and English wire usedin its stead ; the deflection fell to 3 degrees.

This was a great improvement, but not sufficient for rny pur-pose. I commenced to make inquiries about the possibility ofobtaining pure copper, but the result was very discouraging,when, almost despairing, the following thought occurred to me :The action of the coil must be due to the admixture of iron withthe copper, for pure copper is diamagnetic, it is feebly repelled bya strong magnet. The magnet therefore occurred to me as ameans of instant analysis ; I could tell t>y it, in a moment, whetherany wire was free from the magnetic metal or not.The wire of M. Sauerwald's coil was strongly attracted by themagnet. The wire of Mr. Becker's coil was also attracted, thoughin a much feebler degree.Both wires had been covered by green silk ; I removed this,but the Berlin wire was still attracted ; the English wire, on thecontrary, when presented naked to the magnet was feeblyrepelled; it was truly diamagnetic, and contained no sensibletrace of iron. Thus the whole annoyance was fixed upon thegreen silk ; some iron compound had been used in the dyeingof it, and to this the deviation of the needle from zero was mani-festly due.

I had the green coating removed and the wire overspun withwhite silk, clean hands being used in the process. A perfect gal-vanometer is the result ; the needle, when released from the action


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36 APPENDIX TO LECTUKE I.of the current, returns accurately to zero, and is perfectly free fromall magnetic action on the part of the coil. In fact, while wehave been devising agate plates and other learned methods to getrid of the nuisance of a magnetic coil, the means of doing so areat hand. Let the copper wire be selected by the magnet, and nodifficulty will be experienced in obtaining specimens magneticallypure.


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LECTURE II.[January 30, 1862.]

THE NATURE OP HEAT THE MATERIAL THEORY THE DYNAMICAL THEORYTHERMAL EFFECTS OF AIR IN MOTION GENERATION OF HEAT BY

ROTATION BETWEEN THE POLES OF A MAGNET EXPERIMENTS OF RUM-FORD, DAVY, AND JOULE THE MECHANICAL EQUIVALENT OF HEATHEAT GENERATED BY PROJECTILES HEAT WHICH WOULD BE GENERATEDBY STOPPING THE EARTH'S MOTION METEORIC THEORY OF THE SUN'SHEAT FLAME IN ITS RELATION TO THE DYNAMICAL THEORY.

APPENDIX: EXTRACTS FROM BACON AND RUMFORD.

IN our last lecture the developement of heat by mechan-ical action was illustrated by a series of experiments,which showed that heat was easily produced by friction,by compression, and by percussion. But facts alone cannot satisfy the human mind ; we desire to know the innerand invisible cause of the fact ; we search after the prin-ciple by the operation of which the phenomena are pro-duced. Why should heat be generated by mechanical ac-tion, and what is the real nature of the agent thus gene-rated ? Two rival theories have been offered in answer tothese questions. Till very lately, however, one of thesethe, material theory had the greater number of adherents,being opposed by only a few eminent men. Within cer-tain limits this theory involved conceptions of a very sun-pie kind, and this simplicity secured its general acceptance.The material theory supposes heat to be a kind of mattera subtle fluid stored up in the inter-atomic spaces of


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38 LECTURE H.bodies. The laborious Gmelin, for example, in his Hand-book of Chemistry, defines heat to be 4 that substancewhose entrance into our bodies causes the sensation ofwarmth, and its egress the sensation of cold.'* He alsospeaks of heat combining with bodies as one ponderablesubstance does with another; and many other eminentchemists treat the subject from the same point of view.The developement of heat by mechanical means, inas-much as its generation seemed unlimited, was a great diffi-culty with the materialists ; but they were acquainted withthe fact (which I shall amply elucidate in a future lecture)that different bodies possessed different powers of holdingheat, if I may use such a term. Take, for example, thetwo liquids -water and mercury, and warm up a pound ofeach of them, say from fifty degrees to sixty. The abso-lute quantity of heat required by the water to raise itstemperature 10 is fully thirty times the quantity requiredby the mercury. Technically speaking, the water is saidto have a greater capacity for heat than the merctfry has,and this term ' capacity ' is sufficient to suggest the viewsof those who invented it. The water was supposed topossess the power of storing up the caloric or matter ofheat ; of hiding it, in fact, to such an extent that it requiredthirty measures of this caloric to produce the same sensibleeffect on it, that one measure would produce upon mercury.

All substances possess, in a greater or less degree, thisapparent power of storing up heat. Lead, for example,possesses it ; and the experiment with the lead bullet, inwhich you saw heat generated by compression, was explain-ed by those who held the material theory in the follow-ing way. The uncompressed lead, they said, has a highercapacity for heat than the compressed substance ; the sizeof its atomic storehouse is diminished by compression, and

* English Translation, vol. i. p. 22.


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MATERIAL AND DYNAMICAL THEORIES OF HEAT. 39hence, when the lead is squeezed, a portion of that heatwhich, previous to compression, was hidden, must make itsappearance, for the qompressed substance can no longerhold it all. In some similar way the experiments on fric-tion and percussion were accounted for. The idea of call-ing new heat into existence was rejected by the believersin the material theory. According to their views, thequantity of heat in the universe is as constant as thequantity of ordinary matter, and the utmost we can do bymechanical and chemical means, is to store up this heat orto drive it from its lurking place into open light of day.The dynamical theory, or, as it is sometimes called, themechanical theory of heat, discards the idea of materialityas applied to Ireat. The supporters of this theory do notbelieve heat to be matter, but an accident or condition ofmatter ; namely, a motion of its ultimate particles. Fromthe direct contemplation of some of the phenomena ofheat, a profound mind is led almost instinctively to con-clude that heat is a kind of motion. Bacon held a view ofthis kind,* and Locke stated a similar view with singularfelicity. ' Heat ' he says, ' is a very brisk agitation of theinsensible parts of the object, which produce in us thatsensation from whence we denominate the object hot ; sowhat in our sensation is heat, in the object is nothing butmotion? In our last lecture I referred to the experimentsof Count Rumford f on the boring of cannon ; he showedthat the hot chips cut from his cannon did not change theircapacity for heat ; he collected the scales and powder pro-duced by'the abrasion of his metal, and holding them up

* See Appendix to this Lecture.\ I have particular pleasure in directing the reader's attention to an

abstract of Count Rumford's memoir on the Generation of Heat by Fric-tion, contained in the Appendix to this lecture. Rumford, in this me-moir, annihilates the material theory of heat. Nothing more powerful onthe subject has since been written.


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40 LECTURE H.before his opponents, demanded whether they believedthat the vast amount of heat which he had generated hadbeen all squeezed out of that modicum of crushed metal ?1 You have not,' he might have added, ' given yourselvesthe trouble to enquire whether any change whatever hasoccurred in the capacity of the metal for heat by the actof friction. You are quick in inventing reasons to saveyour theory from destruction, but slow to enquire whetherthese reasons are not merely the finespun fancies of yourown brains.' Theories are indispensable, but they some-times act like drugs upon the mind. Men grow fond ofthem as they do of dram-drinking, and often feel discon-tented and irascible when the stimulant to the imaginationis taken away.At this point an experiment of Davy comes forth in itstrue significance.* Ice is solid water, and the solid hasonly one half the capacity for heat that liquid water pos-sesses. A quantity of heat which would raise a pound ofice ten degrees in temperature, would raise a pound of wa-ter only five degrees. Further, to simply liquefy a massof ice, an enormous amount of heat is necessary, this heatbeing so utterly absorbed or rendered ' latent ' as to makeno impression upon the thermometer. The question of4 latent heat ' shall be fully discussed in a future lecture ;what I am desirous of impressing on you at present is, thatliquid water, at its freezing temperature, possesses a vastlygreater amount of heat than ice at the same temperature.

Davy reasoned thus :c If I, by friction, liquefy ice, Iproduce a substance which contains a far greater absoluteamount of heat than the ice ; and, hi this case, it cannot,with any show of reason, be afiirmed that I merely render

sensible the heat hidden in the ice, for that quantity is onlya small fraction of the heat contained in the water.' He* Works of Sir H. Davy, vol. ii,, p. 11.


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FUSION OF ICE BY FRICTION. 41made the experiment, and liquefied the ice by pure friction ;and the result has been regarded as the first which provedthe immateriality of heat.When a hammer strikes a bell, the motion of the ham-mer is arrested, but its force is not destroyed; it hasthrown the bell into vibrations, which affect the auditorynerves as sound. So, also, when our sledge hammer de-scended upon our lead bullet, the descending motion of thesledge was arrested : but it was not destroyed. Its motionwas transferred to the atoms of the lead, and announced it-self'to the proper nerves as heat. The theory, then, whichRumford so powerfully advocated, and Davy so ably sup-ported,* was, that heat is a kind of molecular motion ; andthat, by friction, percussion, or compression, this motionmay be generated, as well as by combustion. This is thetheory which must gradually develope itself during theselectures, until your minds attain to perfect clearness re-garding it. And, remember, we are entering a jungle, andmust not expect to find our way clear.* We are strikinginto the brambles in a random fashion at first ; but we shallthus become acquainted with the general character of ourwork, and, with due persistence, shall, I trust, cut throughall entanglement at last.

In our first lecture I showed you the effect of projectinga current of compressed air against the face of the thermo-electric pile. You saw that the instrument was chilled bythe current of air. Tow, heat is known to be developedwhen air is compressed ; and, since last Thursday, I have

* In Davy's first scientific memoir, he calls heat a repulsive motion,which he says may be augmented in various ways. * First, by the trans-mutation of mechanical into repulsive motion ; that is, by friction or per-cussion. In this case the mechanical motion lost by the masses of mat-ter in friction is the repulsive motion gained by their corpuscles : ' anextremely remarkable passage. I have given further extracts from thispaper in the Appendix to Lecture III.


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42 LECTURE H.been asked how this heat was disposed of in the case of thecondensed air. Pray listen to my reply. Supposing thevessel which contained the compressed air to be formed ofa substance perfectly impervious to heat, and supposing allthe heat developed by my arm, in compressing the air, to beretained within the vessel, that quantity of heat would beexactly competent to undo what I had done and to restorethe compressed air to its original volume and temperature.But this vessel v (fig. 12), is not impervious to heat, and itwas not my object to draw upon the heat developed by my

Fig. 12.

arm; I therefore, after condensing the air, allowed thevessel to rest, till all the heat generated by the condensa-tion had been dissipated, and the temperature of the airwithin and without the vessel was the same. When, there-fore, the air rushed out, it had not the heat to draw upon,which had been developed during compression. The heatfrom which it derived its elastic force was only sufficientto keep it at the temperature of the surrounding air. Indoing its work a portion of this heat, equivalent to the


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FIRE SYRINGE. 43work done, was consumed, and the issuing air was conse-quently chilled. Do not be disheartened if this reasoningshould not appear quite clear to you. We are now in com-parative darkness, but as we proceed light will graduallyappear, and irradiate retrospectively our present gloom.

I wish now to make evident to you that heat is devel-oped by the compression of air. Here is a strong cylinderof glass T u (fig. 13), accurately bored, and quite smoothwithin. Into it this piston fits air-tight, so that, by drivingthe piston down, I can forcibly compress the airunderneath it; and when the air is thus com- Fi - 13-pressed, heat is suddenly generated. Let meprove this. I take a morsel of cotton wool, andwet it with this volatile liquid, the bisulphide ofcarbon. I throw this bit of wetted cotton intothe glass syringe, and instantly eject it. It hasleft behind it a small residue of vapour. I com-press the air suddenly, and you see a flash of lightwithin the syringe. The heat developed by thecompression has been sufficient to ignite thevapour. It is not necessary to eject the wettedcotton ; I replace it in the tube, and urge the pis-ton downwards ; you see the flash as before. If,with this narrow glass tube, I blow out the fumesgenerated by the combustion of the vapour, I can,without once removing the cotton from the syr-inge, repeat the experiment twenty times.*

I have here arranged an experiment intendedto give you another illustration of the thermaleffect produced in air by its own mechanical ac-tion. Here is a tin tube, stopped at both ends, and con-nected with this air-pump. The tin tube is at present fullof air, and I bring the face of my pile up against the

* The accident which led to this form of the experiment is referred toIn the Appendix to this Lecture.


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4r LECTUKE II.curved surface of the tube. The instrument declares thatthe face of the pile in contact with the tin tube has beenwarmed by the latter. I was quite prepared for this result,having reason to know that the air within the tube isslightly warmer than that without. Now, what you are toobserve is this : My assistant shall work the pump ; thecylinders of the machine will be emptied of air, and the airwithin .this tin tube will be driven into the exhausted cyl-inders by its own elastic force. I have already demon-strated the chilling effect of a current of compressed air onthe thermo-electric pile. In the present experiment I willnot examine the thermal condition of the current at all, butof the vessel in which the work has been performed. Asthis tube is exhausted I expect to see the needle, which isnow deflected so considerably in the direction of heat,descend to zero, and pass quite up to 90 in the directionof cold. The pump is now in action, and observe the re-sult. The needle falls as predicted, and its advance in thedirection of cold is only arrested by its concussion againstthe stops.

Three strokes of the pump suffice to chill the tube soas to send the needle up to 90 ; * let it now come to rest.It would require more time than we can afford to allow thetube to assume the temperature of the air around it ; butthe needle is now sensibly at rest at a good distance onthe cold side of zero. I will now allow a quantity of airto enter the tube, equal to that which was removed fromit a moment ago by the air-pump. I can turn on this cock,the air will enter, and each of its atoms will hit the innersurface of the tube like a projectile. The mechanical mo-tion of the atom will be thereby annihilated, but an amount

* The galvanometer used in this experiment was that which I employin my original researches : it is an exceedingly delicate one. When intro-duced in the lectures its dial was illuminated by the electric light ; and animage of it, two feet in diameter, was projected on the screen.


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CONDENSATION OF AQUEOUS VAPOUR. 45of heat equivalent to this motion will be generated. Thusas the air enters it will develope an amount of heat suffi-cient to re-warm the tube, to undo the present deflection,and to send the needle up on the heat side of zero. Theair is now entering, and you see the effect : the needlemoves, and goes quite up to 90 on that side which indi-cates the heating the pile.*

I have now to direct your attention to an interestingeffect connected with this chilling of the air by rarefaction.I place over the plate of the air pump a large glass receiver,which is now filled with the air of this room. This air,and, indeed, all air, unless it be dried artificially, containsa quantity of aqueous vapour which, as vapour, is perfectlyinvisible. A certain temperature is requisite to maintainthe vapour in this invisible state, and if the air be chilledso as to bring it below this temperature, the vapour willinstantly condense, and form a visible cloud. Such a cloud,which you will remember is not vapour, but liquid waterin a state of fine division, will form within this glass vesselR (fig. 14), when the air is pumped out of it; and to makethis effect visible to everybody present, to those right andleft of me, as well as to those in front, these six little gasjets are arranged in a semicircle, which half surrounds thereceiver. Each person present sees one or more of these

* In this experiment a mere line along the surface of the tube was incontact with the face of the pile, and the heat had to propagate itselfthrough the tin envelope to reach the instrument. Previous to adoptingthis arrangement I had the tube pierced, and a separate pile, with its nakedface turned inwards, cemented air-tight into the orifice. The pile camethus into direct contact with the air, and its entire face was exposed to theaction. The effects thus obtained were very large ; sufficient, indeed, toswing the needle quite round. My desire to complicate the subject as littleas possible induced me to abandon the cemented pile, and to make useof the instrument with which my audience had already become familiar.With the arrangement actually adopted the effects were, moreover, solarge, that I drew only on a portion of my power to produce them.


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46 LECTURE n.

jets on looking through the receiver, and when the cloudforms, the dimness which it produces will at once declareits presence. The pump is now quickly worked ; a very fewstrokes suffice to precipitate the vapour ; there it spreadsthroughout the entire receiver, and many of you see a col-

rig. 14.

ouring of the cloud, as the light shines through it, similarto that observed sometimes, on a large scale, around themoon. When I allow the air to re-enter the vessel, it isheated, exactly as in the experiment with our tin tube ;the cloud melts away, and the perfect transparency of theair within the receiver is restored. Again I exhaust andagain the cloud forms ; once more the air enters and thecloud disappears ; the heat developed being more than suffi-cient to preserve it in the state of pure vapour.*

Sir Humphry Davy refers, in his ' Chemical Philos-ophy,' to a machine at Schemnitz, in Hungary, in whichair was compressed by a column of water 260 feet inheight. "When a stopcock was opened, so as to allow the

* See Note (1) at the end of this Lecture.


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FRICTION AGAINST SPACE. 47air to escape, a degree of cold was produced which notonly precipitated the aqueous vapour diffused in the air,but caused it to congeal in a shower of snow, while thepipe from which the air issued became bearded with icicles.' Dr. Darwin,' writes Davy, ' has ingeniously explained theproduction of snow on the tops of the highest mountains,by the precipitation of vapour from the rarefied air whichascends from plains and valleys. The Andes, placed almostunder the line, rise in the midst of burning sands ; aboutthe middle height is a pleasant and mild climate ; the sum-mits are covered with unchanging snows.'

I would now request your attention to another experi-ment, in which heat will be developed by what must ap-pear to many of you a very mysterious agency, and, indeed,the most instructed amongst us know, in reality, very littleabout the subject. I wish to develope heat by what mightbe regarded as friction against pure space. And indeed itmay be, and probably is, due to a kind of friction againstthat inter-stellar medium, to which we shall have occasionto refer more fully by and by.

I have here a mass of iron part of a link of a hugechain cable which is surrounded by these multiple coilsof copper wire c c (fig. 15), and which I can instantly con-vert into a powerful magnet by sending an electric currentthrough the wire. You see, when thus excited, how pow-erful it is. This poker clings to it, and these chisels,screws, and nails cling to the poker. Turned upside down,this magnet will hold a half hundred weight attached toeacn of its poles, and probably a score of the heaviest peo-ple in this room, if suspended from the weights. At theproper signal my assistant will interrupt the electric cur-rent : ' Break ! ' The iron falls, and all the magic disap-pears : the magnet now is mere common iron. At theends of the magnet I place two pieces of iron p P mov-able poles, as they are called which, when the magnet is


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APPARENT VISCOSITY OF MAGNETIC FIELD. 49unexcited, I can bring within any required distance of eachother. When the current passes, these pieces of iron vir-tually form parts of the magnet. Between them I willplace a substance which the magnet, even when exertingits utmost power, is incompetent to attract. Tliis substanceis simply a piece of silver in fact, a silver medal. I bringit close to the excited magnet ; no attraction ensues. In-deed, what little force and it is so little as to be utterlyinsensible in these experiments the magnet really exertsupon the silver, is repulsive instead of attractive.

Well, I suspend this medal between the poles P p ofthe magnet, and excite the latter. The medal hangs there ;it is neither attracted nor repelled, but if I seek to move itI encounter resistance. To turn the medal round I mustovercome this resistance ; the silver moves as if it weresurrounded by a viscous fluid. This curious effect may alsobe rendered manifest, thus : I have here a rectangular plateof copper, and if I cause it to pass quickly to and fro likea saw between the poles, when their points are turnedtowards it, I seem, though I can see nothing, to be sawingthrough a mass of cheese or butter.* Nothing of this kindis noticed when the magnet is not active : the copper sawthen encounters nothing but the infinitesimal resistance ofthe air. Thus far you have been compelled to take mystatements for granted, but I have arranged an experimentwhich will make this strange action of the magnet on thesilver medal, strikingly manifest to everybody present.Above the suspended medal, and attached to it by a bitof wire, I have a little reflecting pyramid M, formed of fourtriangular pieces of looking-glass ; both the medal and thereflector are suspended by a thread which was twisted inits manufacture, and which will untwist itself when theweight it sustains is set free. I place our electric lamp so

* An experiment of Faraday's.


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50 LECTUKE II.as to cast a strong beam of light on this little pyramid :you see these long spokes of light passing through thedusty air of the room as the mirror turns.

Let us start it from a state of rest. You now see thebeam passing through the room and striking against thewhite wall. As the mirror commences to rotate, the patchof light moves, at first slowly, over the wall and ceiling.But the motion quickens, and now you can no longer seethe distinct patches of light, but instead of them you havethis splendid luminous band fully twenty feet in diameterdrawn upon the wall by the quick rotation of the reflectedbeams. At the-word of command the magnet will be ex-cited, and the motion of the medal will be instantlystopped. 4 Make ! ' See the effect : the medal seemsstruck dead by the excitement of the magnet, the bandsuddenly disappears, and there you have the single patchof light upon the wall. This strange mechanical effect isproduced without any visible change in the space betweenthe two poles. Observe the slight motion of the image onthe wall : the tension of the string is struggling with anunseen antagonist and producing that slight motion. It issuch as would be produced if the medal, instead of beingsurrounded by air, were immersed in a pot of thick trea-cle. I destroy the magnetic power, and the viscous charac-ter of the space between the poles instantly disappears ; themedal begins to twirl as before ; there are the revolvingbeams, and there is now the luminous band. I again ex-cite the magnet : the beams are struck motionless, and >theband disappears.By the force of my hand I can overcome this resistanceand turn the medal round ; but to turn it I must expendforce. Where does that force go ? It is converted intoheat. The medal, if forcibly compelled to turn, will be-come heated. Many of you are acquainted with the granddiscovery of Faraday, that electric currents are developed


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HEAT GENEKATED IN MAGNETIC IpSJX 51where a conductor of electricity is set in motion betweenthe

polesof a magnet. We have these currents doubtlesshere, and they are competent to heat the medal. But what

are these currents ? how are they related to the space be-tween the magnetic poles how to the force of my armwhich is expended in their generation ? We do not yetknow, but we shall know by and by. It does not in theleast lessen the interest of the experiment if the force ofmy arm, previous to appearing as heat, appears in anotherform in the form of electricity. The ultimate result isthe same : the heat developed ultimately is the exact equiv-alent of the quantity of strength required to move themedal in the excited magnetic field.

I wish now to show you the developement of heat bythis action. I have here a solid metal cylinder,

1869-Heat Considered as a Mode of Motion [Tyndall]

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