392 SCIENTIFIC THOUGHT, 



c. before the age of Lavoisier, another general conception 



Rule of fixed '^ ' o r 



proportions, had been introduced into chemical research ; this was the 

 rule of definite proportions i.e., the fact that substances, 

 whether simple or compound, combine only in definite 

 proportions of their weight, and that the numbers marking 

 these proportions are characteristic of every definite 

 chemical substance. It took some time, nearly a century, 

 before this idea, which arose through the examination of 

 neutral salts and the determination of the quantities of 

 acids and alkalies which were wanted to effect mutual 

 saturation, became clear ; before the rule of definite pro- 

 portions was generally established, becoming a guide for 

 chemical analysis. It is interesting to note how the 

 vaguer terms of chemical affinity and elective attraction, 

 of chemical action, of adhesion and elasticity mostly 

 borrowed from other departments of science where they 

 had definite meanings gradvially disappeared, when by 

 the aid of the chemical balance each simple substance 

 and each definite compound began to be characterised, 

 and labelled with a fixed number. Nevertheless, even at 

 the beginning of this century, eminent chemists were still 

 so much engaged in discussing the rival claims of the old 

 phlogistic, and the modern theory of combustion, of Ber- 

 thollet's chemical equilibrium, of the so-called dynamical 

 and the electro-chemical views of phenomena, that the first 

 methodical attempt actually to fix these numbers i.e., to 

 give a table of chemical equivalents remained unnoticed.^ 



' The history of chemistry early 

 in this century furnishes a good ex- 

 ample of the sway which theoretical 

 views exercised over the minds of in- 

 vestigators. Berthollet, who began 

 by critically examining Bergmann's 



doctrine of chemical affinities, was 

 evidently much influenced by the 

 mathematical theory of attraction, 

 and by the mechanical laws of equi- 

 librium, which formed so prominent 

 a subject of investigation in the 



