Auo: 27, 1874] 



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SECTION B 



CHEMICAL SCIENCE. 



Openi.si; Address i:y the President, Prof. A. Crum 

 Brown, M.D., F.R.S.E., F.C.S. 

 One hundred years have elapsed since the discoveiy of oxygen 

 by Priestley. Perhaps we should say rediscovery, for there is no 

 doubt tliat about one hundred years earlier Mayow ])repared from 

 nitre nearly pure oxygen, and observed and recorded some of its 

 most marked properties. Mayow's discovery, however, led to 

 nothing, while Priestley's was the most important step in that re- 

 construction of speculative chemistry which was commenced by 

 Black and carried on with surprising energy and thoroughness 

 by Lavoisier and his associates. I shall not detain you Ijy enu- 

 merating the ways in which this discovery has affected chemistry 

 both practical and speculative. The.'pre-eminent position to which 

 oxygen was at once elevated, and which it so long retained, 

 makes this altogether unnecessary. I wish, however, to point 

 ■out one character of the phlogistic controversy which sharply 

 distinguishes it from many others. The truth represented by 

 the theory of Phlogiston was not recognised with sulhcient dis- 

 tinctness by the supporters of that theory to give them any 

 chance of success in opposition to a band of devoted adherents 



of a view which was clearly understood by all. The phlogistists 

 were completed defeated, and the theory ceased to exist. It has 

 been left for chemical antiquaries to pick out, with difficulty and 

 uncertainty, a meaning from the ruins. 



I have mentioned this character because I wish to draw your 

 attention to another more recent controversy, the result of which 

 was very different. 



The questions as to chemical constitution raised about forty 

 ye.-irs ago by Dumas and the new French school, in opposition 

 to Berzelius, may now be said to be practically settled. The 

 great majority of chemists are agreed as to what is to be under- 

 stood by chemical constitution, and also as to the nature and 

 amount of evidence required in order to determine the constitu- 

 tion of a substance. How has this agreement been produced? 

 Some historical writers seem to wish us to believe that it is the 

 result of the triumph of the ideas of Dumas, Gerhardt, and 

 Laurent, and the defeat of the dualistic radical theoiy of Berze- 

 lius ; that the arguments of Berzelius and his followers were only 

 useful as giving occasion for a mora full and convincing proof of 

 the unitary substitution theory than would otherwise have been 

 called for ; that, in fact, the adherents of dualism played the 

 part (not unfrequently supposed to be that of the Conservative 

 party in politics) of checking and criticising the successive deve- 

 lopments of truth, and thus allowing them time to ripen. 



In opposition to the view thus broadly stated, I \\ould place 

 another, and for the sake of contrast shall state it also in perhaps 

 too broad a form : — That the two theories, the dualistic radical 

 theory and the unitary substitution theory, were both true and 

 both imperfect, that they underwent gradual development, 

 scarcely influenced by each other, until they have come to be 

 almost identical in reference to points where they at one time 

 seemed most opposed. 



I have said that the develojiment of the one theory was 

 scarcely influenced by that of the other. Of course the/ziYj- dis- 

 covered by both parties were common property, and the develop- 

 ment of both theories depended upon the discovery of these 

 facts ; but the explanations of facts and the reasoning from them 

 given by each party seemed to the other scarcely worthy of seri- 

 ous consideration, and were treated as matter of ridicule. And 

 tlie habit of mind created by this mode of viewing the opposed 

 theory rendered it difficult for those who were engaged in the 

 controversy on either side to see how nearly the two theories 

 have now come to coincidence. Their language still remains 

 different ; but as the facts are the same for both, it is not difficult 

 for a neutral critic to translate from the one to the other ; and 

 if we do so we shall see that there is much real agreement be- 

 tween the two modes of representing chemical ideas, historically 

 derived, the one from Berzelius, the o'.her from Dumas, Laurent, 

 and Gerhardt. 



In both, chemical constitution is regarded as the oyiLr in vhich 

 iJic constituents are united in tJie compoumt ; and the same funda- 

 mental notion is indicated in the one by reference to proximate 

 constituents, in the other by the concatenation of atoms. To 

 show that this is so, and that the fundamental notion can be 

 arrived at from the dualistic as well as from the unitary starting 

 point, I shall cite an illustrative case. Every student of chemi- 

 cal history will remember the view of the constitution of trichlo- 

 racetic acid propounded by Berzelius, andafterwards supplemented 

 by a similar view of the constilution of acetic acid and an expla- 

 nation of the lil;eness of some of the properties of these two 

 substances. This has .sometimes bten spoken of as a subterfuge 

 of a not vei-y creditaljle kind, by means of which Berzelius 

 apparently saved his consistency while really yielding to the 

 arguments of his opponents. But if, instead of looking at it in 

 the light of the substitution controversy, we consider it in itself 

 as a contribution to speculative chemistry, we at once recognise 

 in it a statement, in Berzelian language, of the views we now 

 hold as to the constitution of these acids. The view was that 

 acetic acid is a compound of oxalic acid and methyl, trichloracetic 

 acid a compound of oxalic acid and the sesquichloride of carbon. 

 They differ considerably from each other, because the " copula; " 

 (methyl and sesquichloride of carbon respectively) are different ; 

 but their resemblance is strongly marked, because they contain 

 the same active constituent, oxalic acid ; and most of the promi- 

 nent characters of the substances depend upon it, and not upon 

 tlie copuhe. Let us first free this statement frum what we may 

 call archaisms of language. It will then assume something like 

 the following form : — The carbon in acetic acid is equally 

 divided between two proximate constituents, one of which is an 

 oxide, the other a hydride of carbon. Trichloracetic acid simi- 

 larly contains an o.xide and a chloride of carbon, between which 



