xiv THE ATOMIC THEORY 299 



Journal, 1802, 1, 272; tr. from Journ. de Physique, 1802, 

 54, 89-95). 



These facts are only partially correct. The artificial 

 sulphide contains 57 parts of sulphur combined with 100 of 

 iron : in iron pyrites there are 114 (not 90) parts of sulphur 

 combined with 100 parts of iron. When iron pyrites is 

 heated it loses one-third of its sulphur, as Proust supposed, 

 but the product is not identical with the artificial compound, 

 ferrous sulphide. * 



Berthollet (1803) challenges the law of fixed propor- 

 tions. In his Essay on Chemical Statics, published in 1803, 

 Berthollet definitely challenged Proust's opinion that in- 

 variable proportions and constant attributes characterise all 

 the true compounds of art or of nature, and that the chemist 

 is no more able to control these proportions and attributes 

 than he is able to control the affinities which the elements 

 possess for one another (Chemical Statics, tr. 1804, II. 

 315-316). Berthollet maintained that the elements 

 can combine in variable proportions, constancy of composi- 

 tion being secured only when some constituent crystallises 

 out, or distils out from the mixture of interacting substances. 

 In the special case of the oxides he proposed, in opposition 

 to Proust, to show 



" that the proportions of oxygen in the oxides depend on 

 the same conditions as those which enter into the other 

 combinations ; 



" that these proportions can vary, progressively, from the 

 term at which the combination becomes possible, to that at 

 which it acquires the highest degree ; 



" and, that when this effect does not take place, it is only 

 because the conditions which I have pointed out become 

 an obstacle to this progressive action " (Chem. Statics, tr. 

 1804, II. 316). 



He suggested that metals like zinc, which oxidise by 

 volatilising, take at once those proportions of oxygen which 

 may be considered as constant ; but that in the case of 



1 The equation, 3FeS 2 = Fe 3 S 4 + S 2 , has already been given on p. 182. 



