PHYSICAL SCIENCE IN THE NINETEENTH CENTURY 361 



have ascertained all these relations, we have to solve the problem of 

 measuring these relations and the laws of mutual dependence that 

 is, of expressing them in numbers. In the first period of chemistry, 

 all the powers of men's minds were devoted to acquiring a knowledge 

 of the properties of bodies. . . . This is the alchemistical period. 

 The second period embraces the determination of the mutual relations 

 or connections of these properties ; this is the period of phlogistic 

 chemistry. In the third ... we ascertain by weight and measure 

 and express in numbers the degree in which the properties of bodies 

 are mutually dependent. The inductive sciences begin with the 

 substance itself, then come just ideas, and lastly, mathematics are 

 called in, and, with the aid of numbers, complete the work. 



QUANTITATIVE RELATIONS : ATOMS ; MOLECULES ; VALENCE. 

 It took . . . nearly a century . . . before the rule of definite pro- 

 portions was generally established, becoming a guide for chemical 

 analysis. . . . 



The vaguer terms of chemical affinity and elective attraction, of 

 chemical action, of adhesion and elasticity . . . gradually dis- 

 appeared, when by the aid of the chemical balance each simple sub- 

 stance and each definite compound began to be characterized and 

 labelled with a fixed number. Merz. 



Proust, analyzing various metallic oxides and sulphides, obtained 

 constant percentage results, from which however no obvious infer- 

 ences could be drawn by him. Dalton (1766-1844) had the happy 

 inspiration to interpret these figures in relation to weights of the 

 combined oxygen, making the lightest element, hydrogen, the unit or 

 measure of his system. His hypothesis that elements combine in 

 weights proportional to small whole numbers the " law of mul- 

 tiple proportions," has since been verified by innumerable analyses. 



It has recently been shown that Dalton was in the habit of 

 regarding all physical phenomena as the result of the interaction 

 of small particles. He was thus naturally led to the conception 

 of definite atomic weights to be determined by experiment. In 

 the words of Dalton : 



We can as well undertake to incorporate a new planet in the solar 

 system or to annihilate one there as to create or destroy an atom of 



