300 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



metals such as tin and lead, which enter into a tranquil 

 fusion, oxidation advances progressively from a minimum 

 towards a maximum, producing a succession of colours and 

 of other properties. Thus lead forms an oxide which begins 

 by being grey ; afterwards it passes to different shades of 

 yellow, and finishes by becoming red. Iron also passes 

 through different shades and acquires different properties as 

 oxidation advances : similar effects may be observed in 

 several metals (loc. cit. p. 317). 



In the case of sulphur, Berthollet expresses the view that 



" The metals can combine with sulphur in very various pro- 

 portions, and the combinations which are thus formed 

 have different properties according to their proportions : I 

 am, in this case, again in opposition to the opinion of 

 Proust, who asserts, that by the invariable law of proportion, 

 sulphur and iron are fixed at ^ " (loc. cit. 372-373). 



Proust (1804-1808) defends the law of fixed proportions ; 

 fixed proportions as a test of chemical combination. 



Proust replied to the criticisms of Berthollet in a series of 

 seven memoirs which appeared in the Journal de Physique 

 during- the last three months of 1804, It is not necessary 

 to follow the controversy in detail : it will be sufficient to 

 notice that Proust finished by using the law of fixed propor- 

 tions as a test to distinguish chemical compounds from mere 

 solutions of one compound or element in another. He was 

 able to appeal to the general feeling of chemical workers 

 to support the idea that there are in fact two different kinds 

 of combination, that the dissolution of sugar in water is 

 something different from the union of carbon, hydrogen and 

 oxygen to form sugar, and so forth. It was no small ser- 

 vice to point out that variable composition and fixed com- 

 position provide a test by which the two kinds of combina- 

 tion can be distinguished sharply from one another. These 

 views were expressed by Proust, in 1806, as follows : 



"The attraction which makes sugar dissolve in water, 



