Manchester Memoirs, Vol. liv. (1910), No. 7. 5 



of the product. So he reasoned. For instance, if hydro- 

 chloric acid is added to a sokition of copper sulphate in 

 water, the copper is divided between the two acids. It 

 was natural to think that all the hydrochloric acid was 

 combined with its share of the copper, and all the sul- 

 phuric acid with its share.- 



Berthollet's attitude is easily misunderstood. He did 

 not so much contradict as transcend the XVIIIth century 

 view. He did not assert that cases of constant composi- 

 tion were non-existent. He admitted, for instance, those 

 of water and ammonia and the oxides of mercury, and 

 was inclined to think that gases combine in constant pro- 

 portion.^ But he thought these instances arose from 

 exceptional circumstances, which he was perfectly pre- 

 pared to discuss. His standpoint was that constant com- 

 position was the exception, and variable composition the 

 rule.^" 



On this matter his great opponent was Joseph Louis 

 Proust. The battle waged chiefly round the oxides of 

 the metals. In the case of a metal which forms more 

 than one oxide, Berthollet held that the oxide at 

 minimum can gradually increase its oxygen without 

 sudden change, till the oxide at maximum is reached. 

 Proust, on his part, while not denying that a metal 

 might yield more than two oxides, directed all his 

 efforts to the study of the two extreme oxides," and 

 showed that material of intermediate composition usually 

 consisted of a mere mixture of these two,'" and 



■* Op. cit., §52. But the instance given above is not Berthollet's. 



" Op. cit., §§ 206, 207. 

 ^" [our. de Phys., vol. 6o, p. ,^47, 1S05. 

 ■■ ' Op. cit., vol 63, pp. 438 — 440, 1S06. 

 '- Op. cit., vol. 55, p. 331, 1802. 



