no 



NA TURE 



[June 14, 1894 



We see then that statement A in no wise discriminates be- 

 tween the views of Proust and Berthollet, and cannot be regarded 

 as an enunciation of the Law of Definite Proportions. That 

 such mi-representation should have arisen, shows how hard it is 

 to find in Proust's writings any sufficiently clear account of 

 his views. I think the following may perhaps be accepted as a 

 correct statement of his position : — 



" If two substances unite chemically, they will do so either in 

 a single ratio or in a series of ratios which are separated by inter- 

 vals of finite magnitude." Berthollet thousjht, on the c )ntrary, 

 that in most cases two substances could combine chemically in an 

 infinite number of ratios varying continuously between certain 

 limits. 



As Berthollet himself pointed out, the whole question turn* 

 on our definition of the term chemiial union, and of compound, 

 which latter we may regard as a substance formed by the chemical 

 union of its constituents. 



The joint efTorts — or rather the divided efTorts— of Berthollet 

 and Proust made clear what neither chemist was willing to 

 recognise frankly, namely, that there exist two distinct classes 

 of homogeneous chemical substances which might fairly have 

 been called compounds : — 



Class I. — The substances belonging to this class are formed 

 by the union of their constituents either in a single ratio or in a 

 series of ratios separated by a definite interval. By the addition 

 of a small quantity of any one of its constituents to a portion of 

 a substance of this class we shall in general obtain a hetero- 

 geneous body. 



Class II. — The substances belonging to this class are formed 

 by ihe union of their constituents in an infinite number of ratios 

 which vary continuously between certain limits. By the addi- 

 tion of a small quantity of any one of its constituents to a 

 portion of a substance belonging to this class, we shall in general 

 be able to obtain a new homogeneous substance. 



Berthollet, on the one hand, regarded the substances 

 belonging to Class I. as exceptions to the normal 

 rule ; Proust, on the other, wished to restrict the name 

 of compound entirely to these. But for such a restriction 

 Proust had certainly no sufficient reason to give. His 

 point, however, as is well known, was carried by the weight 

 of Dalton's discoveries of the Law of Equivalents and the Law of 

 Multiple Proportions, which only applied to Class I. and there- 

 fore drew special attention to it, and by the weight of iJallon's 

 atomic hypothesis, which allowed a sharp theoretical distinction 

 to be drawn between the constitution of substances belonging to 

 Class I. and that of substances belonging to Class II.' 



To enter into a more complete discussion of this distinction 

 as understood by Dalton, and of the modifications of the theory 

 necessitated by our modern ideas on dissociation, would be be- 

 yond the scope of the present letter ; but it should be pointed 

 out that the controversy at present raging on the theory of 

 solutions is, after a long interval, the continuation and develop- 

 ment of the controversy between Berthollet and Proust. 



With the facts clearly set before us, we may now inquire into 

 the origin of the error of the text-books, an error which seems 

 to me to be not a merely verbal one, but one due to a misunder- 

 standing of the real points at issue. 



1 have not found this error in any book before Davy's 

 "Elements of Chemical Philosophy," first published in 1812,- 

 and it appeared but in comparatively few text-books until atten- 

 tion had l>een re-directed to the Law of Definite Proportions by 

 Marignac and .Stas in the "sixties." ' It has since that time been 

 reproduced in most of the books under one form or another. It 

 is often slated, for instance, that Stas showed that " however wc 

 prepare ammonium chloride, it always has the same composi- 

 tion." The teal problem attacked by that great chemist was 

 this: — "Is what we have called 'chloride of ammonium ' a 

 single chloride of ammonium or a series of chlorides of am- 

 monium resembling one another closely, and of which the 

 combination ratios only vary between certain narrow limits?" 

 Or, to stale the matter more generally, " Ought the substances 

 which we have supposed to be members of Class I. I^siipra) to 

 be really considered as members of Class II., with the rcserva- 



' Kopp in hi« contribution 10 ihe much iraniformcd Lchrbuch of 

 " Graham Olio. " and Mendckcf in hi» iruliie, depart very wisely from 

 Moalr,, •, „ ,n oil .. . ■. 'h ihe»e claM«» of hom .gtoeout bodiei coinpounds. 

 , ' '^ : Worki," vol. ir., f.inUi^o). This reference 



''''■■• ■■ ;he Law of Miititple Proportiont includes the 



Law of Ijehoiie Proponioni, and the Ullcr has on thai account often lieen 

 oniilled entirely from t<?«l-books. 



NO. 1285, VOL. 50] 



tion that the limits of the combination-ratios are in these cases 

 close together ? " 



Stas's own language is, it must be admitted, not perfectly 

 unambiguous on this point. 



If Davy's tieatise be possibly the historical source of the 

 error, to trace its intellectual origin, or at any r.ite to offer 

 some plausible explanation of this origin, we must go back 

 further, and consider a time when a nomenclature founded on 

 the exact chemical composition of substances, and the accurate 

 measurement of their properties, was not so much impossible as 

 uncontemplated. 



In those days a common name was given to specimens of 

 homogeneous substances, which might (excluding the case of the 

 elements^ prove to be either 



(i) As in the case of liver of sulphur, a series of substances 

 belonging to Class II. ; or 



(2) As in the case of water, a single substance belonging to 

 Class I. ; or 



(3) As in the case of, say, some particular gun-metal, a /i«^ ' 

 substance belonging to Class II. 



I have chosen the order of these alternatives in thissoniewh.r. 

 arbitrary manner in order to bring out more clearly the point 

 where I believe misconception to have arisen. For I think 

 that what the writers have done is to have considered only 

 alternatives (li and (2), and to have neglected (3). 



After the rise of modern chemistry a closer examination of 

 specimens of "liver of sulphur" and of "water" enabled 

 chemists to say : 



(B) The specimens labelled " liver of sulphur" do not always 

 cont.iin the same elements united in the same proportion. (C) 

 The specimens labelled "water" do always contain the same 

 elements united in the same proportion. 



.And these statements were contracted very naturally into the 

 following : — 



(D) "Liver of sulphur "does not always contain the same 

 elements united in the same proportion; (E) "water "does 

 always contain the same elements united in the same proportion. 



Now the contracted staleuienl (D) has misled the writers into 

 forgetting that " liver of sulphur " is a collective noun, and that 

 the contrast of the two sialements is not a contrast between a 

 mixture ' and a compound, to which it is of course inapplicable; 

 but a contrast between a series of mixtures and a single 

 compound. '- 



They then proceed to substitute for "liver of sulphur" the 

 general term, a ?ni.xlure, and for water the general term, a iom- 

 pound, and so we obtain statement (F) : A mixture does not 

 always contain the same elements united in the same proportion ; 

 which is absurd ; and our old friend (.\): X compound does 

 always contain the same elements united in the same propor- 

 tion ; which bears a very diflferenl meaning to that which its 

 authors intend to convey by it, ami which will be further 

 considered immediately. 



With statements F and A before them it appeared quite 

 natural to the writers to suggest that "ammonium chloride "pre- 

 pared in dilTercnt ways might vary in composition, without their 

 realising that they had returned to the careless days when "liver 

 of sulphur " appeared an eminently satisfactory term.-' 



Perhaps from their error it may be possible to extract some 

 good after all. For would it not be well to state more cle.irly in 

 our books the postulate to which enunciation A reduces itself 

 when interpreted rationally, viz. "Two portions of matter 

 in other respects alike possess the same quantitative com- 

 position. " ^ 'i'his postulate has been tacitly accepted by chemists, 

 and it is made use of every d.iy in the laboratory. 



In conclusion I may be allowed to reply to a criticism which 

 I foresee, namely, th.it no serious misunderstanding has followed 

 from the errors to which I have drawn attention. That maybe 

 so, but accuracy of expression has a value of its own, and my 

 object will have been attained, at any rate partially, if I have 

 succeeded in removing (what should have been) a serious 

 stumbling-block from the path of the student. 



Philip J. Hartog. 

 Owens College, Manchester, June 4. 



* 1 use the word " mixture " for "homogeneous mixture," or " solution" 

 in the sense of V.in't Hoff. 



* They have nedlected nlteni.ltive (3), which would have reminded them 

 thai K applies tjuitc as well ^lo any sini^te homogeneous mixture as 10 any 

 single conipnimd. 



_•' The accurate statement of the problem dealt with by Stas has been 

 gis'en previously. 



* This intcrpretalion may seem to some of my readers to need a fuller 

 explanation, for which 1 have not space here. 



