A CENTURY OF CHEMISTRY. 83 



he was led to adopt the atomic theory in chemis- 

 try in the first instance by purely physical considera- 

 tions, in opposition to the view generally held that 

 the discovery of combination in multiple proportions 

 led him to invent the atomic theory as an interpreta- 

 tive formula. It seems that Dalton, who was not 

 well aware of contemporary continental work, was 

 led to his great doctrine, not by making an induction 

 from his laborious experiments and measurements, 

 but by a deduction from a theory of the constitution 

 of matter which he devised to account for some of the 

 physical properties of gases. As in many other in- 

 stances in the development of natural knowledge an 

 important conclusion was reached deductively and 

 then verified inductively. 



The way in which Dalton reached his conclusion 

 explains why he gave it the extremely generalised 

 form to which we refer when we speak of the atomic 

 theory. While he was thinking about the definite 

 and fixed quantitative proportions observed in chem- 

 ical combinations, he was also experimenting with 

 gases (about 1790), and he had visualised these as 

 consisting of distinct particles : " A vessel full of 

 any pure elastic fluid [that is, gas] presents to the 

 imagination a picture like one full of small shot." 



The idea that bodies are formed of distinct parti- 

 cles was not of course Dalton's, but the chemical ap- 

 plication was. The idea had been suggested in New- 

 ton's Queries, and had been used by Boyle, Boer- 

 have, Higgins, and others; it was indeed one of the 

 legacies with which ancient philosophy endowed 

 modern science. 



Atomic Weights. But Dalton was not content to 

 leave the atomic conception in this vague form, he 

 proceeded, in a manner epoch-making though imper- 



