HISTORY OF THE ATOMIC THEORY. 231 



From the earliest period of his scientific life Dalton had 

 been accustomed to think carefully on the constitution of the 

 atmosphere ; this is seen as early as 1 793, in his meteorology. 

 This subject continued to be a favourite one, and led him to 

 gases generally. The experiments quoted at p. 43, on nitrous 

 gas and oxygen, and those mentioned afterwards in a quota- 

 tion from Dr. Thomson, shew the method by which he came 

 to believe, and to prove experimentally, the existence of 

 definite and constant proportion. 



Here lies the difference between him and Higgins. Higgins 

 expressed the fundamental idea as clearly as Dalton, but it 

 was still left uncertain. Dalton proves it by experiment, 

 draws the conclusion, and tells us the "theory" in a few 

 lines. We have then distinctly the method by which he 

 came to believe in definite proportion and multiple propor- 

 tion. He proved them for himself, and theorized for himself. 

 No books of any writers before him, no Proust or Berthollet 

 controversies were so decisive as these few experiments of 

 Dalton ; not clearer than the words of Higgins, but more 

 decisive, because the result of observation and of i;^asoning 

 combined. This seems to have been his first direct Entrance 

 into the region of the atomic theory. 



In reading over his earlier works, or even in reading the 

 short account here given, we may remark with what a firm 

 grasp he lays hold of the existence of atoms, of the idea that 

 all matter is made up of separate ultimate particles, divisible 

 or indivisible. We find no scientific man holding the idea 

 with such firmness; to others it was a theory, to Dalton it was 

 a fact, which he could not conceive otherwise. We find that 

 even the air is represented* as an agglomeration of bodies 

 heaped up like piles of shot. We appear to be entirely 

 removed from the region of speculation when reading his 

 words; although he leads us farther than the most fantastic 

 speculator had done, the road is made so clear before us, that 



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