LECTURE XV.] HISTORY OF CHEMISTRY. 30! 



Mitscherlich made a similar assumption in the case of antimony 

 pentachloride, 12 and so did Gladstone 13 in that of phosphorus 

 pentabromide. In both of these cases it was assumed that, 

 besides the halogens, the trichloride and the tribromide of the 

 respective elements had been produced. Cahours 14 also ex- 

 pressed the same view in 1847, i n explanation of the low vapour 

 density of phosphorus pentachloride. 



In the same year, Grove 15 made the remarkable observation 

 that water is decomposed into its elements by contact with 

 brightly glowing platinum, a circumstance which he sought to 

 explain as a result of the high temperature. This view met, 

 however, with little acceptance, the fact that platinum can be 

 melted by means of the oxy-hydrogen flame being looked upon 

 as opposed to it. Consequently, Grove's experiment was re- 

 garded as a result of the action of affinity ; and it was explained 

 as exactly similar to the decomposition of water (observed by 

 Regnault) by means of melting silver, where silver oxide and 

 hydrogen were supposed to be formed. 16 



Grove's way of regarding the matter was first definitely 

 proved by Henry St Claire Deville as the result of a very 

 detailed investigation which constitutes the basis of the theory 

 of dissociation. 



Before passing on to describe more minutely these pheno- 

 mena, which are highly important for chemistry, I must here 

 point out that the views with respect to them have been affected 

 by the advances which have meanwhile been made in our 

 knowledge of heat. These advances have been called forth by 

 the law of the conservation of energy, which, as is well known, 

 was first clearly formulated by J. R. Mayer ; ir and they find their 

 expression especially in the mechanical theory of heat and in 

 the kinetic theory of gases, developed chiefly by Clausius, 

 Joule, Rankine, Thomson, Helmholtz, Maxwell, and others. 



After drawing attention to the fact that the affinity of silver 



12 Pogg. Ann. 29, 227. 13 Phil. Mag. [3] 35, 345. 14 Ann. Chim. 

 [3] 20, 369. 15 Phil. Trans 1847, I ; Annalen. 63, I. 16 Ann. Chim. 

 [2] 62, 367. 17 Annalen. 42, 233. 



