Atomic Absorption Spectroscopy — Alan Walsh

Division of Chemical Physics CSIRO

P.O. Box 160 Clayton, Vic, 3168, Australia

When Foil Miller invited me to participate in the Silver Anniversary Symposium on Great Moments in Analytical Chemistry, he suggested that I may care to indulge in some personal reminiscences and com­ments regarding the development and present status of atomic absorption spectroscopy. I hope the ones I have selected may illuminate, if not an­swer, the problem posed by the title of my address.

Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spec­troscopy, Cleveland, Ohio, March 6, 1974. Silver Anniversary Symposium on Great Moments in Analytical Chemistry and Applied Spectroscopy.

I realize that anyone who reminis­ces is usually so decrepit that his memory is totally unreliable. I shall, therefore, try to avoid too many er­rors of fact by restricting my com­ments largely to matters which are documented in reports of CSIRO, in correspondence, or in publications.

My initial interest in atomic ab­sorption spectroscopy was a result of two interacting experiences: one of the spectrochemical analysis of met­als over the period 1939-46; the other of molecular spectroscopy over the period from 1946-52. The interaction occurred early in 1952, when I began to wonder why, as in my experience, molecular spectra were usually ob-

tained in absorption and atomic spec­tra in emission. The result of this musing was quite astonishing: there appeared to be no good reasons for neglecting atomic absorption spectra; on the contrary, they appeared to offer many vital advantages over atomic emission spectra as far as spectrochemical analysis was con­cerned. There was the attraction that absorption is, at least for atomic va­pours produced thermally, virtually independent of the temperature of the atomic vapour and of excitation potential. In addition, atomic absorp­tion methods offered the possibility of avoiding excitation interference, which at that time was thought by many to be responsible for some of the interelement interference experi­enced in emission spectroscopy when using an electrical discharge as light source. In addition, one could avoid problems due to self-absorption and self-reversal which often make it dif­ficult to use the most sensitive lines in emission spectroscopy.

As far as possible experimental problems were concerned, I was par­ticularly fortunate in one respect. For several years prior to these first thoughts on atomic absorption, I had been regularly using a commercial in­frared spectrophotometer employing a modulated light source and synchro­nously tuned detection system. A fea­ture of this arrangement is that any radiation emitted by the sample pro­duces no signal at the output of the detection system. This experience had no doubt prevented the forma­tion of any possible mental block as­sociated with absorption measure­ments on luminous atomic vapours.

In an internal report for the period February-March 1952,1 suggested that the same type of modulated sys­tem (Figure 1) should be considered for recording atomic absorption spec­tra. The following extracts from that report may be of interest:

Figure 1 . Extract from report for Feb­ruary-March 1952

- 2 -

The purpose of this report .is to suggest a new technique for recording absorption spectra which offers many interesting possibilities. The method is basically simple and is illustrated in the diagram below.

Assuming that the sample is vaporised by the usual methods, e . g.

flame, arc, or spark, then the emission spectrum is "removed" by means

of the chopper principle . Thus, the absorption spectrum of the vapour

is measured by passing through it white light which is chopped. The

absorption spectrum and the emission spectrum are then scanned by a

detector, the output from which is amplified by an amplifier tuned to

the same frequency as the chopper. Thus the emission spectrum produces

no output signal and only the absorption spectrum is recorded.

For analytical work it is proposed that the sample is dissolved,

and then vaporised in a Lundegardh flame . Such flames have a low

temperature (2000°K) compared to arcs or sparks (500CTKJ and have the

advantage that few atoms would be excited, the great majority being in

the ground state . Thus absorption will be restricted to a small number

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Report

Stagnant or Pregnant?

"Assuming that the sample is va­porised by the usual methods, e.g., flame, arc, or spark, then the emis­sion spectrum is 'removed' by means of the chopper principle. Thus the emission spectrum produces no out­put signal and only the absorption spectrum is recorded."

"For analytical work it is proposed that the sample is dissolved and then vaporised in a Lundegardh flame. Such flames have a low temperature (2000°K) compared to arcs and sparks (5000°K) and have the advan­tage that few atoms would be excited, the great majority being in the ground state. Thus absorption will be restricted to a small number of tran­sitions and a simple spectrum would result. In addition, the method is ex­pected to be sensitive since transi­tions will be mainly confined to those from the ground level to the first ex­cited state."

At this stage I was thinking of elec­trical discharges, as well as flames, as a means of atomization. It will also be apparent that initially I had not appreciated the difficulties which would be involved in recording atomic absorption spectra when using a con­tinuum source.

The next Bimonthly Report, for the period April-May 1952, includes the diagram shown in Figure 2 and de­scribes our first experiment as follows:

"The sodium lamp was operated from 50 cycles/sec. and thus had an alternating output so that it was not necessary to use a chopper. The D lines from this lamp were isolated— but not resolved from each other—by means of a direct vision spectroscope and their intensities were measured by means of a photomultiplier tube, the output from which was recorded on a cathode ray oscillograph. Ampli­fication of the signal was achieved by the A.C. amplifiers in the oscillo­graph. With the slit-width used the signal gave full-scale deflection on the oscillograph screen. A Meker flame

Figure 2. Extract from report for Apr i l -May 1952

was interposed between the sodium lamp and the entrance slit of the spectroscope. When a solution of so­dium chloride was atomised into the air supply of the flame the signal at the oscillograph was reduced to zero. The principle of the method is there­fore established."

In retrospect, such optimistic naiv­ety is quite incredible.

This simple experiment gave me a great thrill, and I excitedly called in John Willis, who at that time was working on infrared spectroscopy and was later to make important contri­butions to the development of atomic absorption methods of chemical anal-

ysis. "Look," I said, " that 's atomic absorption." "So what?" was his reply, which was the precursor of many similar disinterested reactions to our atomic absorption project over the next few years.

My report for June-July 1952 dis­cusses the problems of recording atomic absorption spectra of flames with a continuum source and con­cludes that a resolution of about 0.02 À would be required; this was well beyond the best spectrometer avail­able in our laboratory at that time. The report concluded as follows:

"One of the main difficulties is due to the fact that the relations between

CHEMICAL PHYSICS SECTION

42nd Bimontly Report. April-May, 1952.

C.P. 1/14. Atomic Absorption Spectra

In the previous report the application of atomic absorption spectra to spectrochemical analysis was suggested. The possibilities of this approach have been explored and the results obtained to date are most encouraging. In the preliminary work the apparatus shown below was used.

The sodium lamp was operated from 50 cycles/sec. and thus had an alternating output so that it was not necessary to use a chopper. The D lines from this lamp were isolated-but not resolved from each other-by means of a direct vision spectroscope and their intensities were measured by means of a photo-multiplier tube , the output from which was recorded on a cathode ray oscillograph. Amplification of the signal was achieved by the A. C . amplifiers in the oscillograph . With the slit-width used the signal gave full-scale deflection on the oscillograph screen. A Meker flame was interposed between the sodium lamp and the entrance slit of the spectroscope. When a solution of sodium chloride was atomised into the air supply of the flame the signal at the oscillograph was reduced to zero. The principle of the method is therefore established. No attempt

ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974 • 699 A

Figure 3. Schematic diagram of use of sharp line source to measure peak absorption

absorption and concentration depend on the resolution of the spectrograph, and on whether one measures peak absorption or total absorption as given by the area under the absorp­tion/wavelength curve."

At this juncture the possibilities of measuring peak absorption were ob­viously coming into consideration.

There is then a gap of four months in my reports, owing to absence from the laboratory on sick leave for much of this period. The next report, for the period December 1952-January 1953, refers to the poor sensitivity ob­tained in the determination of copper by use of a continuum source and a monochromator obtained by placing a slit and detector on the focal wave of a Littrow spectrograph. The report states:

"It is thought that this (poor sensi­tivity) is due to the low resolution of the Littrow spectrograph and to the excessive amount of scattered light at low wavelengths. It is hoped to over­come this difficulty by using a hol­low-cathode source (copper cathode) as source. This will emit sharp lines and a low resolution spectrometer will then be sufficient. The first at­tempts at producing hollow-cathode sources have not been successful."

This use of a sharp line source to measure peak absorption is illus­trated schematically in Figure 3. In this case, the function of the mono­chromator is to isolate the required line for measurement from all other lines emitted by the source. The high

resolution required for atomic absorp­tion measurements is, in effect, pro­vided by the sharp line source.

At this stage we had arrived at the principle of the technique which, in due course, became the generally ac­cepted method of making the intensi­ty measurements required in atomic absorption methods of chemical anal­ysis.

These early experiments, carried out in collaboration with John P. Shelton, were originally confined to hollow-cathode lamps through which argon was flowed by a closed circulat­ing system. We did not commence the development of sealed-off hollow-cathode lamps until December 1953-January 1954, when we first became aware of the work of Dieke and Cross-white (2), which had been published in 1952.

The first person to express any in­terest in the application of the tech­nique we had developed was John David, at the CSIRO Division of Plant Industry in Canberra; and on February 24th, 1953, he wrote to me a letter which began as follows:

"I understand from several sources that you have in mind a new tech­nique of spectrochemical analysis in­volving the measurement of the ab­sorption by the analysis line of an el­ement in a vaporized sample rather than the measurement of its emission.

"I would very much appreciate any information which you may have and are prepared to give me regarding its application to analysis for traces of

metallic and semi-metallic elements in plant ash, soil, mineral or similar samples."

My reply, dated 27th February 1953, was as follows:

"At the moment my work on atom­ic absorption spectra is still in the development stage and I cannot spec­ify exactly what equipment will be necessary, but I think it will include the following items :-

(a) Monochromator having a reso­lution of 1 À.

(b) Discharge tubes and gas-circu­lating system.

(c) Flame burner assembly. (d) Photomultiplier plus associated

power packs. (e) Amplifiers having two homo-

dyne rectifiers on the output side.

(f ) Ratio-meter, potentiometer or ratio-recorder.

(g) One or two choppers. "It is still too early to make any

definite claims for the method, but it certainly offers most exciting possi­bilities and will, I believe, prove par­ticularly valuable in your work, pro­vided the sample can be taken into solution. It may prove possible to ex­tend the method to solids.

"I should add that we are most anxious not to divulge any informa­tion to people overseas, so please re­gard this letter as confidential."

In view of the almost complete lack of interest in our work over the next few years, this request for secrecy was quite superfluous.

Later in 1953 I discussed possible commercial exploitation of our ideas with various instrument manufactur­ers in the United States and England, but the only person who viewed our work with enthusiasm was A. C. Menzies of Hilger and Watts Ltd., London; CSIRO arrived at a tentative exclusive licence agreement with that firm, based on the patent which we lodged in November 1953.

The next significant event was the first public exhibition of a working atomic absorption spectrophotometer (Figure 4). It was demonstrated in March 1954 in Melbourne University as part of an Exhibition of Scientific Instruments, arranged by the Austra­lian Branch of the (British) Institute of Physics.

The apparent complexity of the in­strument was due largely to its being of the double-beam type, which in our early experiments we regarded as essential because of the poor stability of many of our hollow-cathode lamps. The viewer was possibly further con­fused by the optical path being in op­posite directions on the instrument and on the explanatory diagram! Whatever the reason, the instrument aroused no interest whatsoever during the three days it was on exhibition.

700 A • ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974

However, when Dr. Menzies visited Melbourne shortly afterward to assess its performance, he was sufficiently impressed for his firm to decide to produce, under licence to CSIRO, the first commercial atomic absorption spectrophotometer.

As soon as our final patent specifi­cation was lodged on October 21, 1954 (3), I submitted to Spectrochimica Acta my first paper (4), in which I discussed the factors governing the relationship between atomic absorp­tion and atomic concentration, and the experimental problems involved in making atomic absorption mea­surements. The paper was published early in 1955, at about the same time as the paper by Alkemade and Milatz (5), who had independently arrived at the atomic absorption method. Nei­ther paper created any great impact, and Alkemade and Milatz did not pursue their work further, possibly because they regarded this method merely as one for determining "all metals usually to be determined by flame photometry."

In 1956 and 1957 we published pa­pers (6, 7) describing results obtained with our instrument, but these papers also created little interest. John Shel-ton wrote to me from London on March 5th, 1956, after giving a lec­ture on our work to the Institute of Physics, and reported that my first paper had given the impression that "the method was a scientific curiosity rather than a practical analytical method."

When I gave a series of lectures on the subject at the Louisiana State University Symposium in 1958, the net result was that only one person, Jim Robinson, was stimulated into activity. He became a "hot gospeller" and in due course played an impor­tant part in stirring up interest in the United States.

The surprising thing is that the ap­pearance in 1958 of the papers by Allan (8) in New Zealand and David (9) in Canberra did not arouse any sizable impact, even though they de­scribed eminently successful appli­cations of the technique.

On the commercial side, Hilger and Watts had produced an instrument which did not incorporate a modu­lated source and therefore could not fully exploit the technique. Other in­strument manufacturers subsequently perpetrated the same error. By 1958 there was no sign of any instrument manufacturer willing to produce the type of instrument which we thought desirable. This was most curious since by that time there was some in­terest by other Australian laborato­ries in possible applications of this technique. It was at this stage that we decided to arrange for the produc­tion of appropriate equipment in

Figure 4. Photograph of atomic absorption spectrophotometer demonstrated at Institute of Physics Exhibition, Melbourne, March 1954

Figure 5. Photograph of simple atomic absorption spectrophotometer pro­duced commercially in Australia

Australia. The necessary items were manufactured by three small com­panies in Melbourne and then assem­bled by the user, according to our in­structions. As it transpired, for the next few years the members of our re­search group were increasingly in­volved in supporting the commercial production in Australia of atomic ab­sorption equipment. That a new type of Australian industry was eventually created was, of course, cause for much satisfaction, but it was inevita­ble that there was a substantial re­duction in our research effort over a period of several years.

Figure 5 shows a typical instrument produced in this manner, the elec­tronic units having been produced to our specifications by a firm in Mel­bourne called Techtron Appliances Pty. Ltd., which at that time had a

total staff of five. During the period 1958-62, some 30 Australian laborato­ries were equipped by these "do it yourself" units.

While knowledge of the technique spread rapidly throughout Australian industry, there was one memorable exception. I recall the technical direc­tor of one of our biggest mining com­panies phoning CSIRO Head Office in the early 1960's and stating that he had just returned from South Africa where they were using a brand new instrument called an atomic absorp­tion spectrophotometer. He wanted to know if there was anyone in CSIRO who knew anything about it. Our man in Head Office said he didn't know but he would make inquiries!

In this period there was also a slightly increased interest by manu­facturers in other countries. For me,

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one of the "great moments" was in 1962 when I described to various members of staff of the Perkin-Elmer Corp. in Norwalk the impressive re­sults which were being obtained by the laboratories in Australia which were by that time successfully using the technique. It was during these discussions that Chester Nimitz asked, rather tersely: "If this goddam tech­nique is as useful as you say it is, why isn't it being used right here in the United States?" My reply, which my friends in Norwalk have never al­lowed me to forget, was to the effect that he would have to face up to the fact that, in many ways, the United States was an underdeveloped coun­try! The Perkin-Elmer decision to embark on a large-scale project relat­ing to the production of atomic ab­sorption equipment was made shortly afterward. They were, in fact, guilty of overreacting, as witnessed by their subsequent claim that atomic absorp­tion was "the greatest invention since the bed."

It was also in 1962 that Techtron decided to market a complete spec­trophotometer. Initially, this incorpo­rated an imported monochromator, Australian production of an appropri­ate monochromator being delayed until 1965. By that time the ruling engine constructed in the CSIRO Di­vision of Chemical Physics was in full operation and has since supplied all the master gratings required by Tech­tron. Other firms also were becoming increasingly interested, and from that time onward I do not think the tech­nique has suffered any major setback. In 1965 its future was virtually se­cured by the development (down-under) of the nitrous-oxide flame which made the technique applicable to more than 65 elements. I might add that the development of this flame by workers in atomic absorp­tion spectroscopy was most altruistic, since it put back on its feet emission flame photometry which, at that stage, appeared to be dying rapidly.

The recent remarkable growth in the number of atomic absorption spectrophotometers produced is shown in Figure 6 and is indicative of the increasingly widespread accep­tance of atomic absorption methods. In view of such growth, there would appear to be little justification for de­scribing the subject as having been stagnant at any stage during the past decade. But the growth in the num­ber of applications has resulted main­ly from fairly straightforward exten­sions of a technique which originated more than 20 years ago. I believe it is no exaggeration to state that more than 99.99% of all analyses are still carried out by that original tech­nique. In this respect the subject has been stagnant over an extended peri-

Figure 6. World sales of atomic absorption spectrophotometers

od. The situation is particularly de­pressing in view of the well-known limitations of existing flame methods, of which the most serious was de­scribed as follows in our first paper (3) describing our spectrophotometer:

"By far the most serious difficulty in the absorption method is due to the difficulty in atomizing various el­ements. This problem of complete at-omization of the sample seems to us to be the outstanding problem at the present t ime."

As Allan (10) stated in his review of the subject in 1962, "It still is."

In a review (11) I presented in that same year, I pointed out that when a flame is used some elements are only partially atomized, "thus resulting in loss in sensitivity and the possibility of chemical interference due to varia­tions of the degree of atomization of one element with the concentration of other elements, radicals or com­pounds in the solution . . . . This type of interference is present to the same extent in emission and absorption methods and is responsible for serious limitations in flame methods."

Several years later, exactly the same statement was being made, as if it represented the disclosure of a new fundamental truth regarding flame spectroscopy.

It is not my intention to decry the use of a flame as an atomizer, since it is unlikely that any other method will match its enormous range of applica­tion, its speed of operation, or its re­markable convenience. Nevertheless, I remain convinced that the best

means of extending the range of ap­plication of atomic absorption meth­ods of analysis is by developing new methods of atomization.

In some respects I imagine the above remarks present a gloomy pic­ture of the present state of atomic ab­sorption spectroscopy. Fortunately, there is a happier way of assessing the situation, and I want to mention briefly some aspects which lead me to think that this stagnation is more il­lusory than real. In the first place, so-called flameless methods of atomiza­tion, based on developments of the L'vov furnance, have in the last two years become of rapidly increasing importance, and their full potentiali­ties have by no means been fully ex­ploited. They now provide a remark­able ability to analyse extremely small amounts of material, and their importance, particularly in biochemi­cal and clinical work, is already ap­parent. The technique is still in its infancy, and we can expect it to de­velop rapidly over the next few years. Of particular interest to me was the paper presented at the Toronto Con­ference last year in which Segar and Gonzalez {13) described, I believe for the first time, the coupling of a gas chromatograph to the graphite cu­vette of an atomic absorption spectro­photometer. This combination may well make even more ubiquitous the remarkable techniques of chromatog­raphy.

The recent development of im­proved methods for the operation of electrodeless lamps seems likely to

704 A • ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974

produce a new interest in the flame fluorescence methods since, as was first pointed out by Alkemade (14) and demonstrated by Winefordner and Vickers (15), they have striking po­tential advantages over atomic ab­sorption spectroscopy. In this respect, it has always surprised me that work­ers in flame fluorescence have, in general, failed to exploit the fact that the fluorescence phenomenon pro­vides its own monochromator. In the arrangement shown in Figure 7, for example, if the illuminating source is "pure," the only need to have any wavelength selection is to avoid ex­cessive noise owing to radiation from the atomic vapour.

I would also like to refer to our re­cent work on the development of atomic absorption and atomic fluo­rescence methods for the direct anal­ysis of solids (16, 17). The sample is made the cathode of a low-pressure discharge, and the atomic vapour is produced by cathodic sputtering. The encouraging results obtained in the analysis of low-alloy steels have re­cently been published (17). In that paper we reported the difficulties en­countered when the method was ap­plied to the analysis of aluminum-and zinc-base alloys. These difficul-

MODULATED ATOMIC SPECTRAL LAMP

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Figure 7. Schematic diagram of nondispersive atomic flame fluorescence spectrophotometer

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706 A • ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974

ties have now been overcome, and it seems t ha t the sput ter ing technique will be applicable to the analysis of a wide range of meta ls and alloys. It re­mains to compare the performance and range of these methods with those of other analytical techniques.

In the meant ime, I would not ex­pect t he scientific ins t rument manu­facturers to be greatly interested in the simple sput ter ing cell shown in Figure 8.1 would, however, like to think t ha t some of them are musing on possible ways of embell ishment to ensure t ha t any commercial version will have an impressive price tag.

We are now exploring the extension of our work to the vacuum ultraviolet for the determinat ion of carbon, phosphorus, and sulfur. We also pro­pose to extend our experiments to the analysis of powders and solutions. We are increasingly conscious of the im­portant advantages of cathodic sput­tering over thermal methods of a tom-ization in isotopic analysis. Finally, I believe our recent work on cathodic sputtering takes us one s tep nearer the goal I discussed in my first paper on atomic absorption spectroscopy, the development of absolute methods of spectrochemical analysis.

I t would appear, therefore, t h a t the subject has not really been s tagnant , bu t merely pregnant , and has now given bir th to new offspring on which I t rus t Bunsen and Kirchhoff will look with approval and regard as wor­thy descendants of their original brainchild.

References (1) G. Dieke and H. M. Crosswhite, J.

Opt. Soc. Amer., 42, 433 (1952). (2) Australian Patent Application

23,041/53 (Nov. 17, 1953). (3) Australian Patent Specification

163,586 (Oct. 21, 1954). (4) A. Walsh, Spectrochim. Acta, 7, 108

(1955); Erratum, ibid., ρ 252. (5) C. T. J. Alkemade and J. M. W. Mi-

latz, Appl. Sci. Res., B4, 289 (1955). (6) J. P. Shelton and A. Walsh, Proc.

XVth Congress IUPAC, 2, TV-50, Lis­bon, 1956.

(7) B. J. Russell, J. P. Shelton, and A. Walsh, Spectrochim. Acta, 8, 317 (1957).

(8) J. E. Allan, Analyst, 83, 433 (1958). (9) D.J.David, ibid., ρ 536. (10) J. E. Allan, Spectrochim. Acta, 18,

605 (1962). (11) A. Walsh, Proc. Xth Colloquium

Spectroscopicum Internationale, ρ 127, Spartan Books, Washington, 1962.

(12) B. V. L'vov, Spectrochim. Acta, 17, 761(1961).

(13) D. A. Segar and J. G. Gonzalez, paper presented at Fourth International Conference on Atomic Spectroscopy, To­ronto, Canada, 1973.

(14) C. T. J. Alkemade, Proc. Xth Collo­quium Spectroscopicum Internationale, ρ 143, Spartan Books, Washington, 1962.

(15) J. D. Winefordner and T. J. Vickers, Anal. Chem., 36, 161 (1964).

(16) Β. Μ. Gatehouse and A. Walsh, Spectrochim. Acta, 16, 602 (I960).

(17) D. S. Gough, P. Hannaford, and A. Walsh, ibid., B28, 197 (1973).

Figure 8. Photograph of atomic absorption spectrophotometer incorporating sputtering chamber for atomization of solid samples

Alan Walsh is assistant chief of the Division of Chemical Physics, Com­monwealth Scientific and Industrial Research Organization (CSIRO), Melbourne, Australia. He received his DSc from Manchester Univer­sity in England. He is considered the "father" of atomic absorption spectroscopy and exhibited a com­plete apparatus for the technique in March 1954. His now classic paper appeared in Spectrochimica Acta in 1955. He holds basic patents in Aus­tralia, the U.S., and other countries on atomic absorption spectroscopy, multiple monochromators, and im­provements in grating monochroma­tors. One of his major contributions was the development of intense, sta-

ble, and inexpensive hollow-cathode discharge lamps. He has published over 60 papers in atomic, infrared, and Raman spectroscopy. Dr. Walsh is currently interested in what he considers the ultimate goal of spec-trochemical analysis, the develop­ment of absolute methods. This inter­est has resulted in the development of methods of atomization by using ca-thodic sputtering; and these, in turn, have led to the development of reso­nance detection and selective modu­lation techniques. More recently, he and his colleagues have adapted these techniques to the development of atomic absorption and atomic fluo­rescence methods for the direct anal­ysis of solid samples.

708 A • ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974