LECTURE XV.] HISTORY OF CHP:MISTRY. 307 



for example, phosphorus pentachloride, 50 ammonium sulphide, 51 

 ammonium carbamate, 52 etc. A lengthy discussion took place 

 between Wurtz on the one hand. 53 and Troost, 54 Deville, 55 and 

 Berthelot 50 on the other, regarding the nature of the vapour ob- 

 tained from chloral hydrate. This discussion ended in favour 

 of the former, and led to the proof of the decomposition of 

 chloral hydrate upon vaporisation. 



Returning now to the definitions of our fundamental con- 

 ceptions (compare p. 297), we designate as the equivalent, or 

 better, the equivalent weight, that quantity of an element or 

 of a radical which can replace or combine with one atom of 

 hydrogen. This conception, however, no longer plays any essen- 

 tial part ; another one, which stands in close relationship to it, 

 having been introduced instead of it, that, namely, of valency 

 or atomicity. By this term we understand the quotient obtained 

 by dividing the atomic weight by the equivalent, and it was 

 discussed at length in the preceding lecture. The question as 

 to whether the valency of any given element is constant or 

 variable is one of particular importance. So long as we are 

 satisfied to formulate and to make use of our conception of 

 valency in harmony with the above definition, constant valency 

 may, of course, be assumed. As soon, however, as we compare 

 (as it is necessary that we should do) the valencies of the multi- 

 valent elements with one another, we 'can no longer assert the 

 absolute constancy of the valency of any element. Even in 

 the case of carbon, where the assumption of a uniform quadri- 

 valence encounters relatively few exceptions, the existence of 

 carbonic oxide is at variance with its universal accuracy. We 

 find a similar thing, only to a greater extent, in the cases of the 

 other elements, and are therefore obliged to admit the pos- 

 sibility of exceptions in every case. Two different methods 



50 Cahours, Ann. Chini. [3] 20, 369; Deville, Comptes Rendus. 62, 

 1157. 51 Horstmann, Annalen. Supplementband 6, 74. 52 Naumann, ibid. 

 160, i. 5:? Comptes Rendus. 84, 977, 1183, 1262, 1347 ; 85, 49 ; 86, 1170 ; 

 89, 190, 337, 429, 1062 ; 90, 24, 118, 337, 572. Ibid. 84, 708 ; 85, 

 32, 144, 400; 86, 331, 1394. 55 Ibid. 84, 711, 1108, 1256. 56 Ibid, 

 84, 1189, 1269; 85, 8; 90, 112, 491. 



