WILLIAM PRQUT (1785-1850) SAMUEL GLASSTONE Boston College, Chestnut Hill,Massachusetts
WILLIAM PRoUT was
born id the small village of Horton in the County of Gloucestershire, England, on Januarv 15,1785. Most of his life was spent in London where he practiced as a physician until his death on April 9,1850. Although actively engaged in the profession of medicine, Prout yet found time to devote himself to experimental researches in physiology and chemistry. Among his scientific contributions may be mentioned the proof that the acidity of the stomach juices is due to hydrochloric acid (1803) and the identification of uric acid in the excretion of the boa constrictor (1815).' He also developed methods for the analysis of urine and studied the combustion analysis of organic compounds (1815-27). To t h e chemist, however, the aspect of Prout's work which is of the greatest interest is that concerning atomic weights. This subject presents a number of features of historical and scientific interest, and hence it is worthy of detailed consideration. In 1815 there appeared in Thomas Thomson's Annals of Philosophy an anonymous paperZ entitled "On the relation between the specific gravities of bodies in their gaseous state and the weights of their atoms."' In this paper there were compared the "atomic weights" of various elements and their compounds, derived mainly from measurements of combining proportions, and the densities, relative to that of hydrogen, of these same substances in the gaseous state. The author then points out "many strikmg instances of the near coincidence of theory and experiment," that is to say, of the agreement between the weights 'of equal gaseous volumes and the so-called atomic weights as obtained from chemical experiments. A few discrepancies are apparent, but these are largely removed in a second, also anonymous, publication4 by the same author, "Correction of a mistake in the essay on the relation between the specific gravities of bodies in their gaseous state and the weights of their atoms." In his annual review of scientific progress," Improaem n t s in physieal science during the year 1816, Thomas Thomson, the editor of the Annals of Philosophy, refers to the first of the foregoing publications as a
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"very important paper?" and then proceeds toremark: Though the paper in question is anonymous, several circumstancesensbleme to fixwithconsiderablecertainty on the author: hut, as he chooses to remain for the present con&led, I do not. consider myself as a t liberty to mention his name.
However, shortly afterward, the anonymous author must have given permission for his identity to be revealed, for Thornson: in "Some observations on relations between specific gravity of gaseous bodies and the weights of their atoms" now says, "Dr. Prout in a very valuable paper. . . This is the first, and apparently only, indication that William Prout was the author of the two anonymous papers in the Annals of Philosophy. The real explanation of h.is reluctance to reveal himself is not known, but a clue is perhaps to be found in the opening sentences of the first of these papers.
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The author of the following essay suhmita it to the public with the greatest diffidence; for though he haa taken the utmast pains to axrive a t the truth, yet he has not that confidence in his abilities as an experimentalist as to induce him to dictate to others far superior to himself in chemical acquirements and fame.
Although Prout apparently did not realize it, the essential importance of his work lay in its verification of Avogadro's hypothesis that eqpal volumes of gases, under identical physical conditions, contain equal numbers of molecules. This fact is brought out by Thomson,' also evidently unaware of its implication, who states:
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Dr. Prout .bas endeavored to show that the speoific gravity of any (gaseous) body may be obtaihed by multiplying the weight of its atom by one-half the specific gravity of oxygen gas. This is the same thing as to say that the weight of an atom of every body is always double its specific gravity in the state of the gas.
The "specific gravity" here refers to the density relative to that of hydrogen, so that, in modem language, Prout's contention was that thk molecular weight of any substance is equal t o twice its vapor density relative t o hydrogen. This result follows, of course, directly from Avogadro's hypothesis, provided the molecule of hydrogen may be supposed to contain two atoms. From his own observations Thomson concluded that Prout's rule applied to "the atoms of all the simvle sub.. stances-namely, chlorine, sulfur, azote, carbon, and hydrogenv-but he regarded oxygen as an exception. The reason is to be found in the fact that Thornson adhered to the view that the atomic weight of oxygen was eight. A number of other apparent exceptions, among compounds, were also noted, but these were like-
1 .~ Tt, -han- heen subseouentlv ~ ~ found ~~- - ~ ~-" that rmtilian (and avian) excreta, in general, contain uric acid. a Annals of Philosophy, 6, 321 (1815). Vt should be noted that although Avogadro had already indicated in 1811 the difference between the "atom" and the mmolecnIe,u the importance of the distinction was not appreciated until many years later. The term, "atom," as used in this paper is thus meant to apply to the s m a k s t unit of any substance, element or compound. A n n a l s of Philosophy, 7, 111 (1816). =Ibid., 7, 17 (1816). 478
elbid., 7, 343 (1816). 'Loe. cit., ref. 6.
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wise due to the use of erroneous "atomic," i. e . , molecu- number of volumes condensed into one; or, in other words, the number of the absolute weight of a single volume of the first lar, weights. matter (rpwn % b ) which they contain, which is extremely I n addition t o his unwitting verification of Avomultiples in weight must always indicate mdtiples in gadro's hypothesis, Prout must be given credit for the volume, and vice versa; and the specific gravities, or absolute first reliable value of the density of hydrogen. Because weights, of all bodies in a aaseous state. must be multi~lesof the of the extreme lightness of this-gas a& the difficulty of specific gravity or absolute weight of the first matter (;p& 'LA?) because all bodies in their gaseous st.ate which unite with one obtaining it free from moisture, a direct another unite with reference to their volume. of its density could not be contemplat,ed. Consequently, Prout says? In this somewhat involved argument, Prout was evidently greatly influenced by Gay-Lussac's law of volumes, according t o which gaseous suhxtances unite in terms of simple integral volumes. Nevertheless, the tion. Ammoniacal gas appeared to be the best suitod to my essential conclusion, borne out by the data quoted in the purpose as its specfic gravity has been taken with great care by two papers, is that if the atomic weight of hydrogen is Sir H. Davy. . . . The specific gravity of ammonia;, according to Sir H. Davy, is 0.590164, atmospheric air being 1.000. We taken as unity, the atomic weights of all elements should shall consider it as 0.5902. . . Xow ammonia consists of three be integral. It is this contention, capable of experivolumes of hydrogen and one volume of amte condensed to two mental test, that led to many arguments which convolumes. Hence the specific gravity of hydrogen will he found tinued for over 100 years before the situation was to be 0.0694, atmospheric air being 1.0000. finally clarified by the discovery of isotopes. The possibility of the integral nature of atomic The calculation, using 0.9722 for the density of nit.roweights apparently fascinated Thomas Thomson whose geu, is given in a footnote as follows: comments on the work of Prout have already been menLet z = the specific gravity of hydrogen. Then tioned. In 1813 he remarked1=that if the weight of oxygen be taken as unity, "there are eight, atoms of 32 + 0.9722 = 0,5902 2 simple bodies whose weights are denoted by whole Hence, numbers; namely, oxygen, one; sulfur, two; potassium, five; arsenic, six; copper, eight; tungsten, 1.1804 - 0.9722 = 0.0694 2 = eight; uranium, 12; mercury, 25." An obvious exten3 sion of this observation was that of integral atomic According to the most recent data, the density of weights relative to hydrogen, but because he used an hydrogen relative t o air a t 0%. and one atmosphere of incorrect value for density of this gas, Thomson conpressure is 0.06952. cluded that "if f e pitch upon hydrogen for our unit, the I n his first paper Prout called attention t o a regu- atomic weights of all the atoms will be fractional quanlarit,y in the atomic weights of certain elements, thus: tities, except that of phosphorus alone." The pubAll the elementary numbers, hydrogen being considered one, lication by Prout of a more accurate density of hydroare divisible by four, except carbon, azote, and barytium, and gen, as stated above, evidently caused Thomson to these am divisible by two, appearing therefore to indicate that change his views for in "An attempt to establish the they are modified by a number higher t,han that of unity or first principles of chemistry by experiment" (London, hydrogen. 1825) he adduced new data in fapor of Prout's hyThe fact that atomic weights are multiples of two and pothesis. The atomic weights recorded by Thomson differed, four implies that they are integral, hut it was not until however, from those given by J. J. Beraelius (1818), his second paper, containing the corrections t o his original work, that the idea which has become known as and the latter virtually accused Thomson of adjusting the experimental results so as to support his preconPront's hypothesis was de~eloped.~ ceived ideas. "This investi&ator," said Berzelius,I3 If the views are have ventured to advance be correct, we may "belongs to that very small class from which science almost consider the ~ r p & '6Aq'O of the ancients to be realised in hydrogen; an opinion, by the by, not altogether new." If we can derive no a:lvantage whatever. . .and the greatest actually consider this to be the case, and further consider the consideration whirh contemporaries can show to the 8pecific gravities of bodies in their gaseous state to represent the author is to treat this work as if it had never appeared."
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Loe. eit., ref. 2. Loe. eit., ref. 6. 10 This refers to "protyle," the hypothetical primordial suhstance of which some Greek philosophers considered all mrttter to be ultimately composed. " The reason for the qualification of this opinion as being "not altogether new" is by no means obvious. As far as the present writer is aware, no author prior to Pmut had openly made the suggestion that hydrogen might be identified with "protyle." The statement may perhaps indicate another aspect of Pront's diffidence, but J. C. Gregory, in "A Short History of Atomism," A. and C. Black Ltd., London, 1931, p. 143, expresses the opinion
that Prout may have had in mind the views of his illustrious contemporary and compatriot, Humphry Davy, who had heen so successful in achieving the decomposition of several alkalies and alkaline eart,hs. Davy apparently thought many of the so-called elements were capable of decomposition, but that hydrogen was a true, indestructible element and hence might represent the primary mstt,er of which other !'elements" were composed. These views, although not published, may have been current in London scientific circles a t the time (1815-16) Prout was writing his papers. Annals ojPhilosophy, 2, 114 (1813). lS Jahresbe7ich1, 6, 77 (1827).
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Because of the high reputation of Berzelius' work and the confirmation by others of some of his published atomic weights, Prout's hypothesis fell into disrepute and i t was renounced by some of its former supporter^.'^ Among the atomic weights quoted by Berzelius was that of 12.25 for carbon, but when the careful work of J. B. Dumas and J. S. Stas16changed this value t o 11.97, which is very close to an integer, interest in Prout's idea was revived. Upon the termination of his experiments on atomic weights, Dumasr8 concluded "that the equivalents of the elements are often integral multiples of the equivalent weight of hydrogen taken as unity." An outstanding exception t o this rule was provided by chlorine, but since its atomic weight was close t o 35.5, Dumas, following a n earlier suggestion of T. J. Pelouze,17 suggested that for certain elements "the unit with which they must be compared is only 0.5 of the equivalent of hydrogen." A similar modification of Prout's hypothesis was considered by J. C. MarignacL8 who thought of it as an ideal rule, somewhat in the same way as Boyle's law represents the behavior of an ideal gas, the deviations from which demanded careful investigation. It is of interest t o note in this connection that Prout was himself inclined in later years t o accept some modification of his original idea. I n a communication t o Charles Daubeny of Oxford, printed in the latter's "An Introduction t o the Atomic theory" (1831), Prout says :
declared?l "On da't wnsiderer le loi de Prout comme une pure illusion."22 I n the course of his Stas Memorial Lecture, Professor J. W. Mallet2' of the University of Virginia described a private conversation which he had with Stas in the latter's home in Brussels in the summer of 1887. Mallet pointed out that although the atomic weights of many elements were not integral, there were, nevertheless, numerous instances in which the values were very close t o whole numbers, and "having urged upon him (Stas) the improbability of this near approach to integer values for so many atomic weights being due t o chance alone," Stas replied, "I1 faut croire pu'zl y a quelgue chose la-desso~s."~' The fact that so many accurately determined atomic weights approximated to whole numbers also impressed the Hon. R. J. Strutt (later Lord Rayleigh, 4th Baron).%6 He showed that the number of elements with atomic weights that were almost integral is more numerous than would be expected from the laws of chance. "The atomic weights tend t o approximate t o whole numbers far more closely than can reasonably be accounted for by any accidental coincidence," and "the chance of any such coincidence being the explanation is not more than 1in 1000." The views of Mallet and of Strutt, as just outlined, indicate the general opinion of scientists in the early years of the present century. It was felt that the number of elements with approximately integral atomic ' The original opinion to which I was led by the observation of weights was larger than could be accounted for by others, and innumerable experiments (never published) of my chance alone, yet it was an undoubted fact that the own, was, that the combining or atomic weights of bodies bear atomic weights of many elements deviated considerably certain simple relations to one another, fkuently multiple, from whole numbers, e. g., chlorine35.46, copper 63.57, and consequently that many of them must necessarily be multi- %inc85.38, and mercury 200.6. . with the discovery of ples of some one unit; but as the atom of hydrogen, the lowest with isotopes and the development of the mass spectrograph body known, is frequentlysubdivided when in oxygen, etc., there seems to be no reason why bodies still lower as a means for determining their masses, the explanation in the scale than hydrogen (similarly, however, related to one of the discrepancy became a p p a ~ e n t . ~ Most ~ of the another, as well as to those above hydrogen) may not exist, of elements whose atomic weights are close to integral, on which other bodiea may be multiples, without being actually the basis of 0 = 16.000, consist of a'single isbtope, but multiples of the intermediate hydrogen. the others are in general mixtures of isotopes, each of I n view of the recent developments in the field of iso- which has an atomic mass approximating to a whole topes, as described below, it is t o be regretted that number. Chlorine, for example, was found t o consist Prout decided t o change his mind even t o this limited of two isoto~es.with masses of 34.989 and 36.987. reextent!. spectively. -Although the atomic weights of isotopes When J. S. Stas1*commenced his classical stildies of are not exactly integral, the difference from integers, atomic weights in 1841, he was a definite believer in the throughout the whole series of isotopes is so small as to Prout hypothesis, for he said: "Je le dis hautement, be remarkable. The largest discrepancy observed is lorsque j'ai entepris mes recherches, j'avais une confiance about 0.1 atomic weight unit for the 120 isotope of tin. presque absolue duns l'exactitude du principe de P r o ~ t . " ~ ~Thus the history of atomic weights has turned a full Upon the completion of the work, however, Stas was circle, and the principle enunciated by Prout, on the convinced that atomic weights are not integral and he basis of largely inaccurate data, has received remarkable confirmation in an unexpected manner. Hydrogen, '1 Far example, Edward Turner, Phil. Trans. Roy. Soe., 123, however. can no longer be exclusivelv identified with 523 (1833).
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C h a . et Phys., 3, 5 (1841). Ibid., 55, 141 (1859). Aetes soc. helv. sn'. nat., 64 (1860). Amhives (GBnbve), 9 , 105 (1860). ' 9 Bull. Acad. Roy. Belg., 211 (1860). a "I state it frankly, when I undertook my researches I had an almost complete confidence in the correctness of Prout's hypothesis." 1-rn
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""One must consider-the iaw bf Prout to be a complete illusion.'' 'V. Chem. Soc., 63, 1 (1893). "It seems that there must be something in it." Phil. Mag., 1, 311 (1901). F. W. Aston, ibid., 39, 611 (1920); 40, 628 (1920), etc.
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matter. The nuclei of atoms are now believed t o consist of protons, i. e., positively charged hydrogen nuclei, and of neutrons, which have almost the same mass but are uncharged. The weights of all atoms are thus approximately integral multiples of that of hydrogen (and the neutron). The small deviations'from exact integral values are of fundamental importance in connection with the problems of nuclear.stability, but their consideration lies beyond khe scope of this article. I n discussing Prout's hypothesis, E. von Meyer, in his "Geschiehte der Chemie," states that "the predilection shown by many chemists for this conception, which led to such far-reaching deductions, helped to discredit the whole atomic doctrine in the minds of thoughtful investigators."" I n the opinion of the present writer, this judgment is unduly severe. There is little, if any, evidence that Prout's ideas had any retarding idiuence on the development or acceptance of the atomic concept of the structure of matter. In fact they stimulated experimental work on the determination of atomic weights, and some of the most accurate measurements E
" G. MoGowan, 2nd ed., Maernillan and Co. Ltd., London, 1898, p. 203.
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in this field made during the 19th century were undertaken largely for the purpose of testing the hypothesis of integral atomic weights. I n any event, the main purpose Of Prout's papers, as indicated by their titles, was to call attention t o the relationship between "atomic weights" and the relative densities of gases. If the implications of this result had been fully appreciated, the important ideas, enunciated by Avogadro in 1811, might not have lain dormant for nearly 50 years until they were revived by Cannizzaro in 1858. I n the opening paragraph of Prout's pape1,~8part of which has been quoted earlier, he expressed his hope that the "importance (of the essay) will be seen, and that some one will undertake to examine it and thus verify or refute its conclusions. If these should be proved erroneous, still new facts may be brought t o light or old ones better established by the investigation; but if they should be verified, a new and interesting light will be thrown upon the whole science of chemistry." Not merely were these words prophetic; they constitute an expression of the attitude of a true scientfChernistry,"tto his work. Lac. ctl., ref. 2.
