LECTURE XI.] HISTORY OF CHEMISTRY. 1 99 



tion with chlorine and potassium respectively that the silver and 

 oxygen acquire the polarity necessary for their combination. 

 He writes : 



According to Faraday, 4 perfectly dry calcium carbonate is 

 not decomposed even at the highest temperatures, whereas in 

 the presence of water the decomposition begins at once ; and 

 similarly, according to Millon, 5 sulphuric anhydride can be 

 distilled over carbonate of potash, the formation of a salt only 

 taking place on the addition of water. Brodie writes : 



KG + HSO 4 = HO + KSO 4 . 



In this case, especially, it can be distinctly recognised why 

 Brodie considers that combination is always accompanied by 

 decomposition, whilst the first example seems to justify the 

 converse statement. But the existence of free elementary 

 atoms is incompatible with this, so that Brodie is at pains to 

 show that these always appear in pairs, and then unite with one 

 another. The most striking of the examples which he adduces 

 is that of the evolution of hydrogen which takes place when 

 copper hydride, discovered by Wurtz, 6 is treated with hydro- 

 chloric acid : 



Cu 2 H + HC1 = C + u,Cl + HH. 



That a similar evolution is not observed on treating the 

 metal with the acid is looked upon as owing to the fact that 

 the same species of polarity is always associated with the 

 hydrogen in hydrochloric acid, whilst the affinity of copper for 

 chlorine is not sufficient to decompose hydrochloric acid. 7 



3 H = i, O = 8, K = 39, Ag=io8, C = 6, etc. 4 The statement is 

 now regarded as erroneous ; compare Gay-Lussac, Ann. Chim. [2] 63, 

 219. 5 I have been unable to find this fact, quoted by Brodie, in 

 Millon's papers. 6 Annalen. 52, 256. 7 Compare Wurtz, Lecons de 

 philosophic chimique, 64. 



