I)ec., 1911.1 HARRISON--“ THE ELEUTRICAL THEORY OF .DYEING.” 279

solutions always appeared more or less opaque according to the degree of their fluorescence whereas in dichroic solutions that was not so The facts in this instance favoured the solid solution theory, because silk, which in its physical struct)ure, transparency, &c., resembled a liquid or jelly, more than cotton or wool showed fluorescence to a greater degree than other fibre.

Mr Port said he was very grateful for Mr. Prank’s suggestion. that being the kind of information required, because obviously if hie own criticism held good it must have some bearing on the seat of the dye in a dyed fibre, and might have some bearing on the fact that with certain mixtures of dyestuffs they could produce fluorescence on one kind of fibre, whereas with the same dyestuffs on another fibre they could not. In the case of dyed cotton, there was good reason to believe that the scat of the colour in the dyed fibre was different to that in the case of dyed wool, and if evidence could be adduced which would clearly prove the positions which the colour took up in the fibres, they would be in a position then to explain fluorescence obtained by subsequent treatment with different colours. The shot effect on a dyed fabric was not necessarily due to fluorescence in the fibre ; they could get a shot effect hy a fine weave of different coloured warp and weft. This effect, if not the same thing a8 fluorescence in fibres and liquids, was very similar. A proof that the successive treatments with two different dyes left the colour in different> part8 of the fibre and thereby produced a similar fluorescent effect on a fabric to that obtained by weaving fine warp and weft of different shades would enable us to understand fluorescence much better.

The Lecturer, referring to the fact that, whereas a dichroic solution was always of the same transparency, a fluorescent solution ap- parently increased in opacity with an increase of fluorescence, said that was easily explained, because fluorescence was entirely dependent upon reflection a t the surface, and reflection a t the surface was a measure of the opacity of a body. Therefore the more fluorescent a body, the greater the reflection of light a t its surface and the greater its opacity.

A cordial vote of thanks was accorded to the lecturer on the motion of Mr. Ernest Hickson, seconded by Mr. J. C. Oxley.

MANCHESTER SECTION. .-

Meeting held Friday, October 20tl1, 1911, Professor E. KNECHT in the chair.

The Electrical Theory of Dyeing. WILr,IAnr HARRISON, Assoc.M.S.T.

The phenomenn concwnetl in mordantitip ;tnd rlyeing have received niiwli attention, and inany theories have been put forward to explain tlieni, but it cannot be said that any theory put for-

ward up to the present explains all the facts known.

The “ chemical ” theory, which is niainly supported by the experiments of E. Knecht, does not appear to hold for direct colours on vegetable fibres.

The “ mechanical ” theory, favoured by Georgievics and otthers, is not generally accepted, for the fact that porcelain and glass beads will abRorb basic colours is no proof of purely mechanical action.

The “ surface tension ” or “ adsorption ” theory has been shown by Lewis (Phil. Mag. , 6, 16, p. 499, 1908) to be untenable, as, according to it, an increase in temperature should produce R decrease in the amount of colour absorbed, which is not the case in dyeing. Further, there have been many results obtained which are not in accordance with the formula given for ad- sorption.

The “ solid solution ” theory fails to explain many of the important facts known about dyeing.

The ‘‘ electrical ” theory of dyeing appear8 to hare been first proposed by Perrin in a paper on cont,act-electricitv (Jour. de Chim. Ph?p., p. 648, 1904, and p. 100, 1905). This t h e o ~ is based on the following facts :-(u) Any two bodies placed in contact become oppositely charged with electricity ; (b) bodies charged with opposite kinds of electricity attract one another; (c) bodiw charged with the same kind of electricity repel one anothor.

In mordanting and dyeing the bodies which come into contact are the fibres and the solu- tions, which are usually aqueous. The particles or ions of the dye or mordant are electrically charged, and are attracted or repelled by the charge on the fibre.

In 1905, V. Henri and Larguier des Rancels (C.R. Soc. de Biologic: LIX.; 1905, p. 132) used this theory to explain some experiments on the dyeing of gelatine.

In the same year Linder and Picton, in a paper on “ Solution and Pseudo-Solution ” (J .C.S. , pp. 1931 to 1936, 1905) regarded dyeing as a phase of colloidal coagulation, and described substantive dyeing as taking place in two stages :-(I) Coagulation stage, in which single ionic interchange takes place between fibrc wbstance (colloid) and the dye, resulting in the aeparation of insoluble dye derivatives retaining R feeble charge. (2) The colour absorption stage, in which the coagula produced in stage (1) sttract and retain the oppositely charged partioles of the dye subatance.

In 1906 Bayliss (Rioch. Jour., 1, pp. 176-232, 1906) carried out some work on adsorption phenomena with filter paper and direct and basic colours. He came to conclusions similar to those of Linder and Picton, but omitted atage (1) and replaced t’he words “ coagula produced in stage (1) ” by “ colloids or fibre wbstance.”

I n 1908 Larguier des Bancels (Rev. Qeib. Mat. C‘oZ., p. 193, 1908) showed the similarity between

280 HARRISON-" THE ELECTRICAL THEORY O F DYEING." [lkc., 1911.

the electrical coagulation of colloids and the niordanting of fibres.

In the same year Pelet-Jolivet (Rev. Gen. M a t . Col., p. 97, 1908) showed how the electrical theory could be applied to the explanation of the problems of dyeing. In 1909 (R.G.M.C., pp. 68 and 257) lie published papers showing the parallelisms between the laws of dyeing, colloidal coagulation, capillary ascension in absorbent materials, and contact electricity.

In the same year Larguier des Hancels (Comp. Rend., 149, pp. 310-319, 1909) examined the elec trical contact charge of fibre!, and showed that wool, silk, and cotton, when placed in water, took a negative charge, which was in- creased by alkalies, and decreased by salts and acids, and in the latter case the charge was sometimes reversed.

Schwalhe (nip Chemie der Cellulose, pp. 127-128, 1910) does not considcr the electrical theory to be applicable in the case of direct colours on cellulose, or that it offers any ex- planation of the fixation of the dye.

In 1910 Gee and the author carried out some work on this subject, the results being published in the Truns. Faratla?/ Soc. * (April. 1910), an abstract of which appeared in this Journal (dune, 1910, p. 160). One point not given in that abstract was that the maximum negative cliarge of the fibres cotton. wool, and silk was attained a t about 40" C. The remark- able similarity between this experiment and the experiments of Brown (this Joumal, p. 9$, 1901) 011 the absorption of basic colours by wool, was pointed out, Brown having shown that, more colour was absorbed a t about 40" C. than a t other temperatures. This similarity is shown in the curves. Fig. 1 shows the potential difference obtained with wool a t different temperatures ; Fig. 2 the absorption of Metliy-

0 10 20 30 40 50 (I0 70 110 w 100 TEhlPERAlITRE.

Pig. 1.

lene Blue by wool a t different temperatures. Thia is a verification of Brown's work ; t!ie Methylene Rlue absorbed was titratrd with titanous chloride.

Tt, was shown in the above-mentioned paper that substances of a basic nature, such ti

magnesium and aluminium hydroxides and basic colours, took a positive charge when placctl in water. Substances of an acid character. sucli as tnnnic, silicic, rosolic, and stearic acids tool; a negative charge ; direct, acid, sulphide, and vat colours. Alizarin, sulphur, China clay, charcoal, paraffin wax, wool, silk, cotton, and many other substances took a negative charge.

The dyeing of wool with basic colours was explained as being due to the electrical attraction between the negatively charged fibre and the positively charged basic colour.

When wool is plared in an acid solution it becomes popitively charged, and from this fact the dyeing of acid colours was explained as being due to the attraction between the positively charged fibre and the negatively charged acid colour, and the restraining action of sodium sulpliate to its diminishing effect on the positive charge of the wool. The dyeing from colloidal solutions, in which there was no ionisntion, was also considered ; the negatively charged fibres were shown to absorb positively charqecl particles from a colloidal sctlution.

In the discussion which followed the reatling of this paper Dreaper and Feilmann overlooked the remarks on dyeing with colloidal solutions, and took it that dissociation wae considered as a necessary factor in dyeing, which was not the case.

This paper was criticised hy Dreaper ( t l k Jownal, 1910, p. 230), who stated tlitbt the theory fails to explain the dyeing of direct colours in alkaline solution. In referring to a statement made in reply to the discussion, i.c., that " the dyeing with basic colours is carrirtl out in practice a t 90" to 100" C., not with the object of obtaining the greatest absorption of colour, but to obtain bright, even, arid fast shttdes, for which purpose tlie slower the nbsnrp- tion the better," he stated that) " this may come as a shock to many practical dyers." Most dyers are aware of the truth of the above state- ment, which is similar to that made by Brown in 1901. If there are any practical dyers shocked it must be because, in spite of Brown's work, they still think more colour is absorbed at 100" ( '. than a t 40" C. In that case thcy had better try it.

Dreaper further objected to the sta <L t enient that " tho difference between acid end direct colours is more physical than chemical, the frec acids of the former being soluble and t!iose o f the latter being generally insoluble in water."

That statement was made after a careful study of Green's tables of tlic organic colouring matters. Out of 225 direct colours 200 arc stated to give a precipitate with dilute hydro- chloric acid, and of the acid colours 60 per cent. are stated to give no precipitate. Of the re-

Dec., 1911.1 HARRISON-" THE ELECTRICAL THEORY OF DYEING." 28 I

METHOD 4. in electrostatic units.

1()9-- '

C=48.11. .

K 'P Or Cf=48.n* ~~ (3 lo'o)z ev Ev in volts

in Ohms

Putting K=82 and n=0.01134, a t 15" C. this becomes :-

K r P

ev= 1.17 x lo6 xconductivity in mhos ~ - ~-

Pressure in cms. mercurv.

niaining 25 direct colours all those which were obtainahle were found to give a precipitate with acid, either macroscopic or microscopic, and of the remaining 40 per cent. acid colours many were found to give no precipitate unless the acid were strong, or after the dye was freed from impurities. The statement is therefore correct as far as can be ascertained.

The methods iised in the following investiga- tions were the same as those given in the paper just mentioned, and details will be quoted from it. "There does not seem to be any direct method of obtaining the value of the potential difference between hodies placed in contact. We can only avail ourselves of the classic theory of von Helmholtz, which enables the potential difference between a liquid and a porous dia- phragm to be deduced.

This may be done in four different ways :- (1) Ry measuring the rate of flow of the liquid

throq$ a diaphragm of the substance under electrical stress (electrical enclosmose).

(2) By measuring the mechanical pressure ohtairied by electrical endosmose.

(3) Determining the speed of the solid in tlie form of small particles, when immersed in the liquid, under electrical stress.

(4) Finding the difference of electrical pressure between the two sides of a diaphragm when the liquid is filtered under mechanical pressure.

If e be the contact difference between the solid and the liquid, we have :-

METHOD 3.

levels of the colloidal solution, atter tlic elec-

voltage for a definite time, is read on n milli- metre scale placed behind the tubes. (Fol further particulars see Rzirton Phil. Mag., 11, p. 426, 1906.)"

2R, upper solution NaCI solution of the same electrical conductivity. Voltage used a t ter- minals 110. Distance hetween terminals 23 cms. Hq= l l0=4.8

&=amp~e.-~/1750 solution of Diamine Blue

- 23

e=48.11. V K H

in electrostatic units. ~~

per second. ~

H is t,he st,rengtli of electric field. .

E is the electromotive force pro-

p is the mechanical presliure. ,. ix tile re.qiRtance.~~

Use of Method 3.-Tlie observations were

duced.

( 6

ev=47r.n. V ___ x (300)' when e v and H v are K Hv in volts.

Yutt,ing K=82, and n=041134 a t 15" C. this becomes :-

ev=156-5 X V

ev= 1.565 x 102 x 19.3 x 10-*=0.063 volts.

This is the potent,ial difference between the dye particles and the water, and does not give any idea of tlie quantity of electricity carried by those particles, which is dependent on their size. The contact difference is independent of t,he

4.8

D f the form shown in Pig. 3. The dye or col- loidal solution is introduced in the bend of tho tube, so that pure water is in the upper parts Jf the limbs of the tubes. The difference of

Fig. 3. .. . . - . . .

Where :-K is the dielectric constant. 11 is the coefficient of viscosity. V is the speed of particles in cms.

-~

Distance travelled in five minutes= half difl'er- ence in levels in the two limbs=068 cms. Speed V= 19.3 x cnis. per second.

282 HARRISON--“ THE ELECTRICAL THEOHY Oh’ DYEING.” If )n . . , 1911

The apparatus used for Method 4 has been considerably improved, and the improved form only will be described.

In Fig. 4, A is a glass tube 10 cms. long and 14 cms. diameter, provided with ground glass caps. The cotton or other material was placed in this tube. The glass caps are provided with side tubes to hold t>hernionieters T and T,, and have platinum electrodes scaled in them, so as

v \ \

h

Pig. 4.

to reach very close to tlie material packed in the tube. The lower cap is provided with a glass tube dipping into tlic solution in tlie beaker D. The level of the liquid in D was kept constant by means of tlie bottle R and the tube E. The upper cap was connected by pressure tubing to a resistance vessel R , by which the electrical resistance of the solution was measured ; and then to a vacuum flask F. This flask was con- nected to a manometer M and to a vacuum pump. A valve V was attached for the purpose of letting in air to reduce tlie vacuum when re- quired.

The platinum electrodes in the tube A were connected to a Carpentier ailvered mica con- denser C, (t key K, and a galvanometer G.

For making nieasurements the bottle R was filled with the solution to be used, and this was :rllowed to flow into the beaker D, and the electrical conductivity measuied. The pump WAS then started, and tlie solution pumped through the cotton in the tube A until the electrical conductivity of t’lie filtered solution became constant. The pressure was then ad- justed by the valve V, and the key K pressed into cwntwt with X, so that the condenser C I)ccanic c~hargc~l on nworint of the potential clifferencc bctwccn the electrodes a and b.

This contact was maintained for the same time, i.e., one minute, in all the experiments, and then the key was put into contact with Y, whereby the electricihy stored in t,he condenser waa dis- charged into the galvanometer. The “ kick ” or maximum deflection was observed and corn- pared with the result given tinder the same conditions by a standard cell used in place of the tube A . In this way the voltage between the electrodes a and b was determincd. A series of readings a t various pressures was taken, and the average aheration in voltage for a definite alteration of pressure utletl for tlie calculation.

Example. N / I o(K) sodium phosphate (Na,PO,). Conductivity 7.7 x mhos. Capacit,y of condenser, 20 microfarads. 3,300 divisions of galvanometer scalc= 1.00

volt. T A B L E 1.

PreLWLIre 1)ofloctiou

Mercury. UtilvnnniiietHr. I1 . . . . . . -271

1 0 . . . . . . - I S 0 so . . . . . . ~- $13 30 . : . . . . - 4 40 . . . . . . + x5 50 . . . . . . +I72 00 . . . . . . +25fi 50 . . . . . . + IIPJ 40 . . . . . . + 74 30 . . . . . . - 1 0 so . . . . . . - !Hi 10 . . . . . . - 1x5 0 . . . . . . - - I J

The + sign represents the deflection givcn when the elect,rode (a) is positive, the - sign when it is negat,ive.

The deflect)ion given when there is no pressure is nsually neghve , and is caused by some l h l of polarisation. The difference between tho tleflcc- Lions a t 60 oms. and 0 crns. pressure gives the 3eflection caused by the mechanictll pressure md due to the cont,act. difference. Thin i s 528 Aivisions in the ilbove table, and reprenorit,s 528 = 0.16 volts for 60 cms. p- lesswe. 3300

cnis. of

. > - o

-~

ev= 1.17 x lo5 x ~ 1 6 x 7.7 x 10 2 60

= 0.024 volts. Experiment’s with qmrtz show that, Ill(: C W I -

tact difference iR independent of the size of I , ~ I C

?rains; that is to say, independent ( J f the surface exposed or of the rate of flow of tlic solution.

Coaree quartz, flow 300 cos. wat,er per niinute, with a pressure of 60 crns. mercury, gave a oontact difference (ev) of 0.020 volts.

Fine quartz, flow 4.3 CCS. per minute, wit11 pressure of 60 crns. mercury, gave a contact difference (ev) of 0.021 volts.

Alcohol decreases tlie voltage. 96% ~~Icohol gave a contact difference ( e t ) of 0.0127 volts,

The results in t,lm following t,ahles givc tllc contact’ clitference between cott)on ant1 RoIutions uf various substances.

Dec., 1911.1 HARRI80N--" THE ELECTRICAL THEORY OF DYEING." 283

TABLE 11. N/1000 xolritions, nll on the same siimple of cotton.

Sodium hydrate. . Diarnine Blue 2B Trisodium phosphate Sodiuni sulphate Sodiuin chloride Magnesiuni aulplittte Hydrochloric acid .4lrnnininm snlpliete

. . . . 0.0300

. . .. 0.0287 .. . . 0.0240

. . . . 0'0219

. . . . 0.0183

. . . . 0.0136

. . . . 0fKl85

. . . . 0'0030

TABLE 111. Soclirim Hgtirnto Soliit.ions.

N/2,000 . . . . . . . . 0'0270 N/4,000 . . . . . . . . 0.0217

N/1,000 . . . . . . . . 0.0300 N/500 . . . . . . , . 0'0294 Nj200 . . . . . . . . 0.0248

TABLE J V . Hydrorlilnrir Arid Rolritionrr.

N/10,000 . . . . . . . . 0410x N/4,O(H) . . . . . . . . 0.01 38 Ni'2,000 . . . . . . . . 0.01 11 N/l,i)OO . . . . . . . . 0.0085 NjlOO . . . . . . . . 0 ~ 0 0 5 5

TABLE V. Aluniininrn Sii1phnt.o Soliit,ions.

N/10,000 . . . . . . . . 09092

N/2,(H)O . . . . . . . . 0.005d

N/BOO . . . . . . . . 0~0025

N/Fi,OOO . . * . . . . . 0007R

N/1,000 . . . . . . . . 0.0036

TABLE VI. Croceine Fcnrlot Solutions.

N/S,OOO . . . . . . . . 0.0239 N/2,6(H) . . . . . . 0.0247 N/1,000 . . . . . . . . 0.0234 N/500 . . .. . . . . 0.0220

TABLE VII. Dituninti Rluo Soliitions.

N/5,000 . . . . . . . . om10 NP2.500 . . . . . . . . om?H N/1,000 . . . . . . . . 0.0287 NIAOO . . . . . . , , o w 2 3

TAI3I.E VIIT. Sotlirrin Olonta Soliitionrr.

N/1,000 . . . . . . . . O'O%RA N/500 . . . . . . , . 0.0.28X N/W) . . . . . . . . 0.0317 N/100 . , . . . . . . o m 1 3

It has already bcen shown by Knecht that pure cellulose absorb8 practically no colour from a Rolrition of pure Henzopurpurine. Thic was found to be tole case with most pure dyes. The pure dyes used for this work were prepared from the conimercial ssmples by " salting " out with animonium carbonate, re-dissolving and again salting out,. After washing with a saturated solution of ammonium carbonate, the precipi- tated dyestuffs were dried a t 110" C., when all the ammonium carbonat,e was volatilised off ~

leaving the dye free from salts. For the experiments on dyeing, pieces of

Sclileiclier and Schiill's purest filter paper, 7 cms. diameter, weighing 0.17 gms. and con- taining 040003 gms. ash, were wed. This roughly corresponds to O.02 per rent,. a&. A 6

These pieces of paper were dyed with 1 /100th millimols dye per gm. cellulose (1 millimol equals 1 /1000th molecular weight in gms.), with additions of various salts, in test-tubes, jus t sufficient water being used to cover the paper, i.e., 12 ccs. This represents 70 times the weight of paper. The dye solutions are thus M/7000, or N/1750 in the case of Diamine Blue 2B (4 Na atoms) and N /3500 with Benxopurpurine (2 N.a atoms).

With Diamine Blue 2B in "/I750 solution only a weak shade was obtained, and the addi- tion of one tnolecule of sodium chloride per molecule of dye procluced only a slight increase in the absorption of colour, so that it woultl appear that the dilution rather than the purity nccoiinted for the weak shade obtained. Wheii a stronger solution of the dye wa8 used, the pure rellulose was dyed a deep shade ; the dye being n salt acts like any other salt, and increases the absorption of colour. This effect is the more marked the more soluble the dyestuff is.

With N/1750 solution of this dye tlie absorp- tion of rolour by pure cellulose increased with addition of sodium chloride up to 4,000 equiva- lents per molecule of clye, when there was a decrease.

With N/3500 solution of Benzopurpurine, the amount of colour absorbed increased with addition of sodium chloride up to 1,024 equiva- lents per molecule of dye, and decreased after that strength. In presence of gelatine, which prevents the salting out of the dyestuff, the absorption of colour steadily increaRed with addition of salt.

The effect of various salts on tlie absorption of dyestuffs by pure cellulose was next investi- gated.

With Diamine Blue 2R, Benzopurpurine, and Rosophenine 4B, and N/1760 solutions of various salt8, the shades obtained increased in the order :-NaOH, Na,P04, Na,S04, NaC1, HCI. MgSO,. Aluminium sulphate precipitates the dyestuffs. It will be seen that the above sub- stances lie in the same order as for their de- rrcasing effect on the contact difference between cotton and water, Table TI., except in the case of hydrochloric acid.

With N/175O Benzoparpurine and Roso- phenine the addition of N ,4760 to N 9 7 5 NaOH produced a decrease in the absorption of colour by pure cellulose ; N/440 NaOH gave the same h d e as no addition ; further additions pro- duced an increase in the absorption up to 256 squivalentq per molecule of clye, when a decrease was again produced. These results correspond very closely to the alterations in contact differ- mce with cotton, caused by addition8 of sodium iydrate EIR shown in Table 111. This is sliuwii n the curves, Figs. 5 and 6.

Now in carrying out the experiments on iyeinq, a result was obtained which did not aorrcspond with what w a ~ expected from the -eRults of the experiments on contact difference.

On adding increasing amounts of aluminium rulphate to a dyebath of Diamine Blrie N /1750,

2 $4 BARRISON-" THE ELECTRICAL THEORY OF DYEING." [Dec., 1911.

an increase in the absorption of colour was pro- duced up to N/440 A12(S0J3, but on doubling this strength the absorption decreased. On adding still more Al2(SO,Js, the absorption again increased, and apparently decreased after

N/7. According to the results on the contact difference against c,otton, there should be a gradual increase in the absorption of colour a6 Al,(SO,), is added. On examination, i t W ~ E

N/2000 Nj1000 N/600 Fig. 0.

found that the charge on the dyestuff Waf diminished by the Al,(SO,), until a t N/220 thc dye -carried practically no charge. By furthei additions the charge became reversed, and thir incr eased,

The results are given in the next table :-

TABLE IX. N/1,760 Dinmine Rlue 2B - 00030 volts

,, ,, + N,'1760A12(504)~ -0.0132 ,, 9, 1, + N/875 9 , -00101 ,, ,, ,, + N/440 ,, -00009 ,, ,, ,, + N/220 , I +00005 ., ,. , ,, + N/110 ,. +00038 ,,

At N/220, in acdordance with the electrica laws of coagulation of colloids, the dye is com pletely precipitated, and this accounts for tht decrease in absorption. When more Al,(SO,), ii added the dye gradually becomes re-dissolved ir

the form of a colloidal solution with positively charged particles, which are readily attracted by the negatively charged fibre. From thc experiments on the contact difference with cotton (Teble V.), it is evident that the charge will eventually be reversed as excess of A1 ,(SO,), is added, and when this occurs the dye and fibre will have the same charge, and a decrease in the absorption will follow. This would account for the decrease after N/7.

It is obvioue that the effect of salts on the ohage carried by the dye must be considered, in addition to the effect on the charge of the fibre.

The researches of Perrin, Burton, and others show that as the electric charge on the particles of a colloidal solut,ion diminishes, the size of tlie

articles tends to increase, and when the charge Kecomes zero precipitation takes place. The experiment with Diamine Blue 2B and alu- minium sulphate seems to show that the same applies to solutions of ionised dyestuffs. The process of " salting " out is probably a conse- quence of such electrical neiitralisation.

In experiments carried out with Crystal Scarlet 6R the salting out efiect of different salts was found to increase in the order :-NaOH, Na,PO,, Na,SO,, NaC1, MgSO,, Al,(SO,),, that is, in the same order as in the results on dyeing with direct colours. The last two substances form the magnesium and aluminium salts respect- ively, but thie does not interfere with tlie electrical effects. The effect of hydroohlorir acid was intermediate between that of Na2S0, and Na,P04. Acids generally h w e exceptional effects when compared with salts, but they also have exceptional effects on the contact charge of many substances, as shown in the results of Perrin.

Although hydrochloric acid gives a less con- tact difference with cotton than magnesium sulphate does, the latter has a greater precipi- tating action on Diamine Rlue 2B than the former. This would account for the position of these two Substances in the results on dyeing being reversed in the results on contact differ- ence.

&@hide CoZours.-The effect of salts on the dyeing of sulphide colours was found to be similar to the effect on direct colours. With Immedial Sky Blue, without the addition of sodium sulphide, the order in which the salts were placed was the same as for the direct coloum, but MgSO, and Al,(SO,), precipitated the dyo. With this colour the gradual addition of sodium sulphide first increased the absorption of colour and then decreased it. The same result was obtained with Immedial Rlack NNG. Although the action of the sodium sulphide is undoubtedly chemical as far as trhe dye is con- cerned, it also produces a physical change ; it decreases t;he size of the particles of dye. A similar effect is produced with most colloid^ when tho electric charge is increased by suitable means. The effect of such decrease in tho size of the dye particles will be considered later.

n-., 1911.1 HARRISON-" THE ELECTRICAL THEORY OF DYETNU." 288 - - ___ ______~ - ___ - - __ __ - -

Since the vat colours also have a negative charge in water, i t is to be expected that salts would have a similar effect on the absorption of them by cellulose as with the direct colours, hut no experiments have yet boen made.

Mmdanting with Tannic Acid.-Tannic acid is usually regarded as a colloid which is not ionised in water, and since it carries a negative charge i t should not be absorbed by cotton, according to the electrical theory. But the experiments on contact difference showed that tannic acid had the same effect as hydrochloric acid of the same electrical conductivity. This is in accord- ance with the law of Perrin, which states thnt all mono-valent acids act the same for the same concentration of H ions. Whether the con- ductivity of the tannic acid was due to im- purities or not, cannot be said; the sample contained 96 per cent. trtnnic acid, which was the purest obtainable. However, i t has been found by Srivastava (private communication) that the purer the tannic acid and the cellulose used the less tannic acid is absorbed. In dilute solutions of tannic acid, M/1600 to M/600, the addition of salts and acids had the same effect on the absorption by cellulose as with the direct colours, and the order in which these substances were placed was the same as with those colours. The best result was obtained with acid, which is the usual addition used in the tanning of cotton. Aluminium sulphate precipitated the tannic acid when heated. In all cases the solutions were heated, and allowed to cool slowly, the tannin being fixed by acetate of iron.

Mordanting of Cotton with flodium Aluminate. -With a solution of sodium aluminate contain- ing no excess of alkali, the addition of sodium hydrate decreased the absorption of alumina by cellulose, the addition of salts increased it, and they acted in the order Na,PO,, Na,SO,, NaCl, that is, exactly the same as for direct colours. This was expected, since the alumina in sodium aluminate carries a negative charge. HCl, MgSO,, A12(S0,), produce precipitation.

Absorption of dcid Colours by Cotton.-When pure cellulose is boiled in a solution of Croceine Scarlet 3RX, the dye is absorbed, but is practi- cally all removed by cold water. However, by adding salts to the dyebath, and washing the coloured cellulose five timea, with equal volumes of salt solutions of the same concentration as used in the dyeing (N/100), the shades came out in the order :-NaOH, Na,PO,, Na,SO,, NaC1, HCl, MgSO,, .4l2(SO4),; that is, the same as for the direct colours. This also was expected, as the acid dyes are also negatively charged.

The above experiments show the parallelism between the effect of salts on the contact differ- ence of cotton and on the absorption of dyes and mordants which are negatively charged in water, the order of increased absorption being :- For N/1000 solutions NaOH, no addition, Na,PO,, Na,SO,, NaCl, (HCl), MgSO,, Alz(S04)Y. For solutions of greater concentration than N /400 the first two are reversed. Measurements

~ ~ ~ - __.___

made with cotton treated with various reagents will now be dealt with.

The following facts are known with regard to the effeot of treating cotton with various re- agents on its affinity for direct, sulphide, and vat colours :-

Nitrated cotton absorbs less colour than ordinary cotton.

Cotton mercerised with sodium hydrate, nitric acid, &c., absorbs more colour than untreated cotton.

Cotton mercerised with sodium hydrate under tension absorbs less colour than when mercerised without tension.

Mercerised cotton loses much of its increased afinity for dyestuffs on drying.

Cotton treated with sodium hydrate of mercer- ising idrength, and extracted with alcohol, dyes

veB amples of cotton were treated in the above manners and placed in the apparatus shown in Fig. 4, and boiling water pumped through until no more soluble substances were dissolved out ; this took from 6 to 20 hours. The contact differences against N/1000 NaCl were then determined, with the following results :-

little better than ordinary cotton.

TABLE X. Contact difference

in voltR. 1. Untreated cotton .. .. .. .. 00272 2. Nitrated cotton .. 00602 3. Cotton inexwised i d e r &ion &d dried 00261 4. Cotton treated with 4 0 Tw. NaOH, and

extracted with dcohol . . .. 00214 A. Cotton mermrised without ternion i d dried 00196 6. Cotton mernerieed without eension tested

whilst wet .. .. .. a . 0-0101 7. Cotton mercerieed with nitric acid.. .. 0.0094

On dyeing samples of the above with Diamine Blue 2B the shades obtained increased in the order :-2, 1, 3, 4, 6, 6, 7, so that the deorease in contact charge runs quite parallel with the increase in the absorption of colour.

Acid Colours on Wool and Bilk.-Pelet-Jolivet and Anderson (Roll. Zeit., 2, 226, 1908) found that in the dyeing of wool in acid bath with Crystal Scarlet the acids acted in the order :- H,PO,, HaS04, HCl, which is again in accordance with the theory.

Pelet-Jolivet and Siegrist (Koll. Zeit., 6, 236, 1909) have shown that in the dyeing of wool with Crystal Ponceau the gradual addition of sul- phuric acid produces first an increase, then a decrease after N/O.8-H,SO4.

As already stated, wool takes a positive charge in an acid solution; the acid dye, however, retains its negative charge when the acid is dilute. It is probable that as more acid is added the negative charge of the dye will eventu- ally be reversed, just as the negative charge of Diamine Blue is revemed b aluminium sulphate.

positively charged, and there will be a decrease in absorption as the charges increase.

An experiment on the dyeing of Bilk in alkaline solution has recently been reaorded by Dreaper and Wilson (J.S.C.I., p. 1432, 1910). Starting

When this occurs both I bre and dye will be

28 6 HARRISON-" THE ELECTRICAL THEORY OF DYEING." [Dee., 1811.

.- - -

with a solution of an acid colour they found that the first addition of alkali (sodium carbon- ate) produced a marked derease in the amount of colour absorbed, but with further additions the shade obtained gradually increased in depth. They state, however, that the colour is fixed under conditions which give little upp port to the Perrin electrical theory of dyeing or to a theory which assumes ionisation of the dyestuff. This experiment is exactly analogous ta the experiments on the dyeing of cellulose with Benzopurpurine and Rosophenine in alkaline solutione, already described. Since Perrin has shown that the order in which substances act on the contact charge is independent of the nature of the boundary, it may be concluded that alkali would have the same effect on the contact charge of wool or silk as on that.of cotton. Attempts have been made to prove this, butwere not successful owing to the rapid absorption of alkali by the wool ; a t any rate the experiment above mentioned appears quite in accordance with the electrical theory.

It is known that when wool is boiled with increasing quantities of sulphnric wid the quantity of acid colour absorbed by i t increases.

Experiments show that by the above treat- ment the negative charge of the wool becomes reduced and then reversed. The positive charge appears to inorsass as more acid is used, but the acid becomes gradually washed out, and this inteifered with the el&$rical measurements.

The results already given show the parallelism between the alterations in contact difference and the alterations in dyeing properties, but no explanation has been given for the manner in which the colour is absorbed. For this purpose other experiments muRt be referred to.

It was found that when a solution of sodium chloride was filtered through a column of puri- fied cotton, hydrochloric acid passed through. As the solution was too dilute for chemical analysis, t,he presence of acid was proved by an electrical method ; the solution had a greater conductivity after filtration than before. This effect is probably caused by the contact charge, the negative charge on the fibre causes it to take up tlic positive sodium ions, leaving the positively charged H ionH close to the fibre, to combine with the negative chlorine ions as shown in Fig. 7.

H - N o % + - + - H - N a X +-=I-- H --a% + - + -

H Nr1U Ma + + - H Na@ = NU + + - H Now Na

A corresponding effect was produced when s solution of aluminium qulphate, Methylene Blue, Croceine Scarlet, or Diamine Blue was used ; in all cases the liquid imuing from the :otton contained hydrogen ions, that is to say, it was acid.

H 1

n ge

nu

c(-Naoon +-4- - n - Na OH + - + - - H -"o OH R f - + -

n on n OH

H OH

Fig. 8.

x = Coloiir inn or eolloihl particle.

These experiments are important for oxplain- ing the absorption and fixation of the direct colours. The colour ions of direct colours are negatively charged, the metallic ions positively charged. The cotton is negatively charged, and the water immediately in contact with it positively charged (see Fig. 9). The fibre tliiiR

Fig. 9.

repels the colour ions, but i t attracts the metallic ions. The negative charge on the fibre is reducod by the absorption of pouitive metallic ions ; n t the same time the negative charge on the colour ions is redaced by the positive ions in the water next to the fibre. The repulsion of tlie fibre for the (lye iR thus reduced until i t does not prevent the dye arriving a t the fibre by rirtuc of its molecular movements (or Brownian niove- ment in the case of colloidal solutions). Whcn this occurs the colour ions are absorbed by the fibre, and again increase its negative charge. The process then repeats itself until tlie dilution becomefi too great.

A similar process would occur with all negatively charged fibres in presence of nega- tively charged dyes or mordants. With sulpliidc colours the Na,S provides the + ions.

It has already been shown that the difference between the'acid and direct colours lies in the fact that the latter are precipitated by dilute acids. Now during the process described above, when the Polour ions of the direct colour meet the positive hydrogen ions sot free by the fibre, the dye is precipitated on (Fig. 9) or in (Fig. 10) the fibre. With the acid colours no such pre- cipitation occurs, and the colour is therefore not fixed.

When salts are added to the bath, the charge on the fibre is reduced to a greater extent than when the pure dyes are used. At the same time the salts reduce the charge on the colour particles

Dec., 1911.j HARRISON-'' THE ELECTRICAL THEORY OF DYEING." 287

or ions, and aid in their coagulation. The ab. sorption of colour is therefore facilitated.

To show that the direct colours are coagulated during the dyeing process the dye was mixed with a solution of gelatine, which, being a pro.

Fig. lo.

tective colloid, prevents or reduces coagulation especially with negatively charged substances. The quantity of colour fixed was much reduced the effect being greater the more salts present in the solution.

Now one of the greatest problems in dyeing is to get a " resist '' towards direct colours. From the explanetions above given i t is apparent that to obtain such a resist the fibre must have a negative charge of such magnitude that all the positive ions present in the bath cannot reduce it, below tlie value which repels the dye particles or ions, and prevents them arriving a t the fibre. The best resiRtR known a t present are nitro- cellulorJe and tannate of tin, both having powerful negative charges. The results obtained should he better when few or no salts are present in the dyehath, or when the solutions are dilute.

The absorption of positively charged sub- stances is next considered.

Basic Colours on Cotton.-As already stated, by the filtration of a solution of Methylene Blue through cotton, hydrochloric acid passed through, leaving the cotton dyed. When the clear filtrate was again passed through the dyed cotton most of the colour was removed by the hphchloric acid present. When a sample of cotton was placed in a solution of Metliylene Blue of the same strength as that used in the filtration experiment, the amount of colour absorbed was only small ; but in this case the liberated hydrochloric acid remains in the bath, where i t diminishes the absorption of colour by reducing the charge on the cotton. This accounts for the fact that less colour is absorbed by cotton from a strong solution of basic colour than from a weaker one, as shown by Riltz and Steiner (Roll. Zeit., 1910, 7, 113-122) If the acid is neutralised the colour is readily absorbed by cotton.

With tanned cotton, which has a powerful negative charge, the dyeing process proceeds much further, but stops when the acid liberated yeduceu the negative charge below tho value necessary to cause further absorption. This

free acid has also some effect on the remaining dyestuff, which tends to prevent further ab- sorption.

The same applies to the dyeing of wool or silk with basic colours.

The action of salts on the dyeing of basic coloum should be different from the action on the dyeing of direct colours, and the order in which they accelerate the dyeing should be opposite to that shown for direct colourfl. Pelet and Grand (KoU. Zeit., 2, 83,1907) have shown this to he the case for basic colours on silk. the order being Al,(SO,),, MgSO,, no addihon, NaCI, Na,B04, NasP04, NaOH.

The dyeing of wool with basic colours in an acid bath would follow 8 similar process to the dyeing of cotton with direct colours, Rince both fibre and dye have the same charge, positive in the case in question.

The addition of salts would alter the charge on dye and fibre, but the order in which the substances act would be opposite to that with the direct colours.

Mmdanting with Aluminium Sulp?mte.-The mordanting of fibres with aluminium from a Aolution of aluminium sulphate, in which tlie alumina carries a positive charge, follows the same rules as tfhe dyeing with basic colours. Cotton with a negative charge takes out the positive alumina from a dilute solution of alu- minium sulphate, and sets free sulphuric acid, which reduces the charge on the fibre and pre- vents further absorption. When the acid is neutralised somewhst, which may be effected by making the aluminium sulphate basic, the mordanting process is facilitated. The effect of salts should be the same as with basic colours. Other mordants would follow the rules laid down for alumina in aluminium sulphate, or sodium aluminate, according as the mordant itself carried a positive or negative charge.

The dyeing of such mordanted cotton with alizarine colours follows from the attraction between the positively charged mordant and the negatively charged alizarine colours. The action of the lime usually added in the dyeing of mordanted cotton with alizarine is to reduce the size of the particles of dye. The effect of this will be considered below.

The results already given show that as the electric charge on the fibres beoomes less negative or more positive the attraction for negatively charged ions or particles of dyes or mordants increases, whilst the attraction for positively charged particles deoreases.

The fixation of the colour can occur by three processes :-(a) Electrical attraction, as between positive fibre and negative dye (wool dyed in acid bath with acid colours), or negative fibre (or absorbent) and positive dye (wool, tanned cotton, glass wool, china clay, tc. , dyed with basic colours). (b) Coagulation of the dye on or in the fibre consequent on tlie reduction or

288 HARRlSON--“ THE ELECTRICAL THEORY OF DYEING.” tbec., 1911. -~ - -~ ~

destruction of the electric charge carried by the clye particles by the charges on the fibre. This latter is assisted by the presenco of salts (cotton dyed with direct colours wit,li and without the addition of salts). (c) Comhination of (a) and (b) (dyeing of cotton, wool, or tanned cotton with a colloidal solution of Night Blue baae) (see paper in Trans. Far. BOG.). The fastness of the shades obtained will depend on the manner the colour is fixed.

In all the cases dealt with up to the present the surface of the fibre exposed to the action of the dye has been considered as remaining con- stant, so that the alteration in the contact difference represents the alteration in the quantity of available electricity. Rut if the oontact difference were constant, an increase in the surface of fibre exposed would produce an increase in the quantity of available electricity, and a corresponding increase in the quantity of colour absorbed. (Precipitated celhilose absorbs more colour than ordinary cellulose.) (Dinitrocellulose dyes like ordinary cellulose, but when made into the form of a film, absorba little or no colour.) (Weber, J.B.C.I., 1894,

If the surface exposed increased whilst the contact charge decreased, the quantity of avail- able electricity would increase or decrease according as the increase in surface produced a greater or less effect than the decrease in contact difference.

Considering the dye in a similar manner, a8 the quantity of electricity carried by the particles of dye decreases, moro clye will be required to carry sufficient electricity to the fibre to neutralise its charge ; the amount of dye absorbed will therefore increase.

There are complications in the relation be. tween the potential of the contact charge of thc particles of dye and the quantity of eleotricit) carried by them. The potential governs the ratt of absorption and the qiiantity governs the tota substance absorbed.

If the potentialdifference between the paiticler of dye and the surrounding solution remainec constant whilst the size of the particles in creased; then, since the total surface of dyt exposed would be decreased, the quantity o electricity carried by a given weight of dye woulc decrease, and the amount absorbed woulc decrease.

In the experiment with aluminium sulphtatc the potential between the dye particles and thc surrounding solution decreases, and the Rize o the particles increases with additions up tc N lZ20, A1,(S04), ; after this strength thl p&entittl increases I , and the size of particle decreases.

With Tegard to the surface of the fibre, onc important point must be noted. The fibres arc porous, and a large portion of the svailablc surface lies within those porcs. Tho whob

p. 120.)

__ - - - __ ~ - -~

,urfaca is therefore only available to particles if dye which are able to enter those poree.

An increase in the size of the particles of dye vould only produce an increase in absorption inti1 the particles approached the size of thc )ores, when a decrease would occur. This wonld xplain the decrease in absorption of Diamine 31ue with N /220 aluminium Rulphate.

The above points having been Considered, iyestuffs were chosen giving particles of in- Brewing size, when mixed wit’h a dilute solution If rutlt. They were placed in the order:- h c e i n e Scarlet 3BX, Diamine Blue IB, Benzo- mrpurine 4B, Rosophenine 4B, Immedial Sky Blue, AliLarin, Immedial Black. The last) two lyes, being insoluble in water, were compared n dilute sodiiim sulphide. Further, the fibres were examined in the ultramicroscope, and it was :oncluderl that they could be placed in the xdeI :-Silk, wool, cotton, for increasing size of pores.

The particles of acid dye can enter ttlie pores 3f each fibre, but in the case of cotton they are jasily removed again, since they are not coagu- lated. The pores of wool or silk would be more :apable of retaining the particles of acid colour. When dyed in an alkaline bath, as clone in the sxperiments of Dreaper and Wilson, the particles 3f Anthracene Acid Red are evidently coagu- lated somewhat, and are thereby retaincd by tho silk fibre.

Going to the other extreme, the particle8 of Emmedial Black or Alizarin are too large to Enter the pores of cotton, and to make this possible the particles must be reduced in size by the addition of sodium sulphide in the former and lime in the latter caee. As inore of these substances are added t tie particles become re- duced in size until they become too small to be easily retained by the pores of t,he fibre. This occurs in the case of sulphide colours-a little sodium nulphide increases the absorption, too much decreases i t ; and also in the case of Alizarin, a little lime facilitates the dyeing, too much retards it.

The fastness of a dyestuff depends on the reagent to which i t is tested ; for this reason a large sample of cotton was dyed with Dianiine Blue 2B and portions boiled in solutions of salts, &c., of the same Concentration, N/B00. The fastness increased in the order :-Sodium oleate, NaOH, Na,P04, Na2S04, NaCI, HCl, MgSO,, Al,(SO,),; that is, in the same order given for the dyeing of tho direct colours. The fastness to water was the same as that to Hodium phosphate solution. Samples were next dyed with the addition of the above salts. and tested for fastness to N/riOO sodium oleate. For this experiment i t was necessary to use pure filter paper, as the salts present in cloth interfered with the results. The fastness increased in the order :-A12(S04)s, MgSO,, HC1, NaCI, Na28O4, Na,PO,, NaOH, sodium oleate, water. From these results I constructed a table (XI.) showing the order of fastness of colourv dyed in ditrcrent

Dee., 1911.1 HARRTSON-" THE ELECTRICAL THEORY OF DYEING." 289 ~- __

ways to different reagents. Experiments madl with cloth seem to verify the above results wherc the numbers exceed 11 no colour wai rernoved. In all mses the fastness was juclgec according to the relative proportion of t'ht absorbed colour removed during treatment wit1 the solutions.

One point stands out clearly-the samplt dyed without) addition of salts was the fastest

It lias already been shown (Tnhle VIII.) thal soap increases the charge on cotton, and thir would cause repulsion of the dye particles and aid in t,heir removal. At the same time by in. creasing the charge on the dye particles thc soap would tend to break them up into mallei particles, and this effect would be the greatei the larger the particles were.

TAI{LE s r . Fasliiea~ ( i f Ditriiiiiic Bliic 213 t o N/BOO Sulutioiis of :-

Additioiix used in Dyeing N/R(W.

-~

None . . Sodiiuni Ole& NaOH .. Na3P04 . . N u ~ S O ~ . . NaCI , . .. HC'I . . .. A12(S04)3 . . hfgS04 . .

I '

7 9 10 11 12 13

-

A

B v

7 __

16 15 14 13 12 11 10 9 8 -

The additions which facilitate the absorption of dye most, coagulate the direct dye most, and produce the largest particles of dye; they should therefore give the shades of least fast- ness. The above experiments show this to be the case.

Now the smaller the particles of dye are, the more they are capable of entering the smallest pores of the fibre ; and the smaller the pores in which the dye iu fixed the more difficult it would be to remove that dye. Any process which aids the dye particles getting into the smallest pores and becoming fixed there would aid in pro- ducing fast shades. Since all salts tend to salt out direct colours nnd form larger particles of dye, the shades should be fastest when dyed without salts. This also lias been shown to be the case. It appears from the results of Dreaper and Wilson that the same applies to Acid Anthracene Red dyed on silk.

Again, since an increase in temperature tends to decrease the size of the particles of dye, it should increase tho fastness of the resulting shades. The fastness of Aniline Black, ingrain cglours, sulphide colours, vat colours, and others is due to the fact that them colours are dyed under conditions which allow them to get into the smallest pores of the fibre before they become precipitated.

The above results show that any attempt to increase the absorption of (t direct colour, Diamine Blue 2B, b j adding salts to the bath, will decrease the fastness ; whether this applies to all direct colour8 remains to be proved. The fastness of baBic, acid, sulphide, and other colours has not yet been investigated. Sulphide and vat colours should be the faster the more reducing agent used, providing that none of the dye becomes destroyed by chemical action.

The results in this paper show that the electrical theory finds considerable application in explaining the problems of dyeing, including those not explained by other theories. There are four points which must be considered :- (a) The charge on the dye particles, (a) the charge on the fibre, (c) the size of the pnrticles of dye, and ( d ) the surface of the fibre, including that within its pores. There are many other problems which can be explained by this theory, but they require further knowledge of the last two factors, and will be left for future investiga- tion.

In considering either the fixation of a dye by 8 fibre or the removal of it, the effect of the various conditions, reagents, &c., on all the %hove four points must be taken into account, md since these factors may be altered differently many complicated effects can be foreseen.

In conclusion, the author desires to thank Prof. Knecht for facilities placed at his disposal, md also Prof. Gee for the methods used in the :lectrical measurements.

DISUUSSION.

Mr. Thompson asked how the Lecturer had nade his electrical measurements, and to what init the figures given in the paper were re- 'erred. In looking a t the patterns submitted, ie had noticed that different sQsngths of dye iolutions had been used. Were the results oom- ,arable in that case ?

Mr. Hubner said that i t was well known to iyers that basic colours were faster when dyed bt 90" to 100' C. He asked whether any experi- nents had been made with regard to the ab- iorption of colour by cotton mercerised without ,ension which had afterwards been premed. l e had observed that pressing decreased the iffiity for dyestuffs. Croceine Scarlet behaved uite abnormally as compared with other acid

fvestufls in its property of dyeing cellulose 40 actual dyeing of the fibre could be observed vith suoh dyestuffs as Naphthol Yellow S, the ?onceaus, and Acid Magenta. He asked whether he statement as to the decrease in the absorption I f basic colours by the action of aluminium ulphate referred to animal or vegetable fibrea. Is far as uellulose was concerned, experience in he dyeing of paper pulp pointed to exactly the Ipposite conclusion. He hoped to bring forward he results of some experiments with regard to he influence of various solv@8 (such as alcohol, rater and alcohol, &c.) on the absorption of

290 HA’RRISON-“ THE BLECTRICAL THEOW OF DYEING.” [hw., 1011

dyestuffs which might be of intereRt in connec- tion with the remarks made by Mr. Harrison.

Mr. Gray asked what qualities of tannin had been employed in the experiments in which it was observed that the purer the tannin the less was absorbed. Had the Lecturer noticed the effect of temperature in mordanting with sodium aluminate ?

Mr, Srivastava stated that in some experi- ments which he had conducted recently the purer tlie tannin the less the amount absorbed by cotton. Up to 5 per cent. sodium chloride he had noticed increasing amounts of tannic acid absorbed, and with more than 5 per cent. the absorption of tannic acid decreased.

Mr. Nair asked how the Lecturer had deter- mined the purity of the cellulose which he used. He understood that the filter paper u,sed con- tained about 0.02 per cent. impurities, and it seemed to him that these might influence the dyeing properties, especially in such cases as that shown by experiment, in the lecture. He questioned the accuracy of the determination of liy’drochloric acid in the solution which had passed through the filter paper. The sohition was too dilute for chemical tests.

Mr. Vlies said the Lecturer appeared to hove ahown that electrioel phenomena occurred during the dyeing of cotton with direct colours, but not much was said about other dyeing processes, He should have liked some information as ta the manner in which the dye was fixed on the fibre, and to know whether i t was a clremical compound or only held in a physical way.

Profefjllor Knecht drew attention to what hr uonsidered an unfair comparison between tlir three salts, sodium chloride, sodium sulphate and sodium phosphate. The Lecturer had evidently chosen them hecause the first repre. aented tho salt of R mono basic acid, the second dibasic, and the third tribesic. It must not bc forgotten, however, that sodium phosphate waa distinctly alkaline in character, and that quit( recently Arndt had shown that sodium sulphatt behaves abnormally a t elevated temperatures Hummel and Gardner were the first to show thal hydrochloric acid had a greatm effect tliar sulphnric acid on the dyeing of wool with acic colours. Hummel also showed that caustic sodr increased the abso tion of basic colours. Therr

of pores in fibres. Moreover, artificial sill (especially that made from gelatine) could no have pores, but this dyed like other fibres. HI agreed with Mr. Huhner in considering tht colour Crooein Scarlet abnormal, and mentionec mother dyestuff of the same character, namely R particular brand of Soluble Blue, formed! sold by ‘Brook,, Sihpson t Spiller, as ‘‘Non Mordant Cotton Blue.” He had shown t h a cotton steeped for three hours in nitric mid o 83’ Tw. showed a marked affinity, not only fo these two, but f o r all acid colours. The expeti IiientB made by the Lecturer in su port of tlic

did not appear to 3 e any proof of the presencc

slwtrical theory might help in exp f aining mmi

henomena observed in dyeing, but the chemical $wry explained them equally well, if not better.

The Lecturer explained to Mr. Thonipson that ie had measuied the contact difference in ex- Ieriments similar to that shown on the lecture ,able. Except in the experiments on fastness, lie patterns shown on separate cards were not ntended for comparison. Each card repre- iented a separate series of experiments. Tn meply to Mr. Hubner, Be did not consider it new D tlie dyer that basic colours were faster when Xyed a t ~0’-10Oo C. ; the statement was made 0 emphasise the fact that maximum absorption iid not correspond to maximum fastness. He lad not made electrical measurements with :otton mercerised and pressed. His reaHoii for hoosing Crocein Scarlet was that it had a slight bffinity for cotton, and he deRired to show that ,here was no essential difference between acid bnd direct colours except that the latter were Drecipitated by acids. Crocein Scarlet was 3tated in Green’s tables to be precipitated by wid, and to this fiwt he attributed the affinity which i t had for cotton. There were other ex- 3eption8, and no doubt the colour mentioned by Prof. Knecht would he found to belong to this ;lass. He found that aluminium sulphate decreased the d eing of batjic rolours on all

basic COIOU~A, and thereby produced more even haderr. His statement with regard to t’lie effect of impurities in tannic acid was quoted Irom tlic results of Mr. Srivastava, who had kindly placed them at his disposal. The cellulose uaetl was Schleicher and Schull’s best quantitative filter paper. Sodium chloride, sulphate, and phos- phate had been chosen for the reason nientioned by Prof. Knecht, but in his experinientR the fairness or otherwise of the comparison did not matter, because they were simply taken to show. the agreement between the effeot of these salts on the contact difference and on the dyeing properties. Ultramicroscopic investigations ap- peared to indicate the presence of pores in artificial fibres. Although he was not entirely opposed to the chemical theory, he should be glad if Prof. Knecht could give liiin some chemical explanation of the effects obtained with aluminium sulphate and caustic soda. He thought the electrical theory explained them very well, but he could not see what clieniical action N/1000 NaOH had on cotton, or what chemical compound was formed when 32 molecules of aluminium sulphate were required to completely recipitate one molecule of Diarnine Blue ; i! the four sodium atoms in the dye were replaced by aluminium, three mole- cules of dye would require two molecules of aluminium sulphate.

Mr. Thompson, in proposing a vote of thanks, mentioned that many chemical reactions were accompanied by electrical phenomena.

Mr. Hiibner seconded this, and mid that lie had still quite an open mind with rogarcl to the theory of dyeing.

fibres. It retar B ed the dyeing of cotton with