LECTURE III.] HISTORY OF CHEMISTRY. 45 



stant proportions ; which, as a matter of fact, he had never 

 wholly denied. In 1809 he still regards compounds with 

 variable proportions as possible, 40 but in this opinion he stands 

 isolated. Too much support has now come to the opposing side. 

 Richter's investigations, carried out from 1791 to 1800, had at 

 length become known ; Gay-Lussac's classical work on the pro- 

 portions by volume in which gases combine was concluded ; 

 Berzelius had published his first important papers ; and Dalton 

 had formulated his atomic theory, which was irreconcilable 

 with Berthollet's view, and was already beginning to constitute 

 the basis of chemical considerations. Thus the controversy 

 ends, apparently with the complete defeat of Berthollet. 



I have treated this subject at length, because I consider it 

 very important. We have here to do with a general doctrine 

 which constitutes one of the foundations of our theoretical 

 considerations, and settles the distinction between mixtures 

 and compounds. It is for the latter alone that our chemical 

 laws hold, since mixtures are not subject to them. In any 

 particular case, therefore, it is very important to know which 

 class of substances is being dealt with. What, then, are our 

 means of forming an opinion ? 



It is to be found stated in text-books, that compounds 

 possess a homogeneous character, whereas mixtures can very 

 frequently be mechanically separated into their ingredients. It 

 is further stated there that in compounds the properties of the 

 constituents have disappeared, whilst they are present side by 

 side in mixtures. Finally the constancy of proportions is then 

 adduced as characteristic, and I wish to direct attention to 

 this whole matter. There are cases in which mixtures are, 

 in their whole behaviour, no longer distinguishable from com- 

 pounds. We then have recourse to analysis to solve the ques- 

 tion. We prepare the substance in various ways, and observe 

 whether it always possesses the same composition. We thus 

 invert the doctrine discussed by Berthollet and Proust. 'The 

 former regarded compounds with variable proportions as pos- 



40 Mem. d'Arcueil. 2, 470. 



