xiv THE ATOMIC THEORY 297 



proportions of the constituent elements of various sub- 

 stances. A similar disregard of quantitative measurements 

 prevailed during the vogue of the phlogiston theory, which 

 became untenable as soon as the balance became a recog- 

 nised tool of chemical science. 



The credit of introducing quantitative conceptions into 

 chemistry belongs mainly to Black, whose work on 

 " Magnesia Alba " was a masterly demonstration of the 

 possibilities of the new method. Black's methods were 

 used with even greater effect by Lavoisier, the whole of 

 whose work was carried out upon a quantitative basis. 



The work of Black and of Lavoisier was based upon the 

 tacit assumption of the LAW OF FIXED PROPORTIONS, that : 



" Chemical compounds are formed by the combination of 

 their elements in fixed proportions by weight." 



This law can be deduced at once from Dalton's theory 

 that " the ultimate particles of all homogeneous bodies are 

 perfectly alike in weight, figure, etc." Conversely, if the 

 law of fixed proportions were not true, Dalton's theory 

 would be a false hypothesis which could not stand the test 

 of experiment. 



Proust (1799-1802) formulates the law of fixed propor- 

 tions. The law of fixed proportions, which had been tacitly 

 assumed by Black and Lavoisier, became a definite doctrine, 

 to be tested and verified by experiment, in the hands of the 

 French chemist Joseph Louis Proust (1755-1826), who 

 carried out much analytical work bearing on this question 

 whilst Professor of Chemistry at Madrid. Proust was im- 

 pressed by the fact that the proportions in which the ele- 

 ments combine are " fixed by nature " and that the " power 

 of augmenting or diminishing " these proportions " is not 

 given to men " (Journ. de Physique, 1802, 55, 325). Proust 

 had already developed this view in 1799, when he found 

 that there was no difference of composition between natural 



