2 Meldrum, Development of the Atomic Theory. 



to theory may exert a harmful effect on science, by 

 despising and rejecting a theory of the utmost importance. 



The usefulness to science of the atomic theory is so 

 completely established now, that it must seem strange to 

 us to observe the efforts Dalton had to make, in order to 

 arouse attention to the importance of his ideas regarding 

 atoms. For some nine years, (1801-1810), if not longer, 

 he endeavoured to spread abroad his ideas, both b}' private 

 communications and publicly, by his writings and by 

 lectures in various parts of the country. 



As will be seen, Dalton's speculations had to encounter 

 dangers of two kinds In the first place; not many people 

 gave themselves much concern about the question of the 

 continuity or discontinuity of matter. They were quite 

 content to go on speaking of ''atoms" and "molecules" 

 in a vague, colloquial sense, and Dalton had to induce 

 them, if possible, to use the words as terms of precision. 

 This done, there was always the possibility that they 

 would reject Dalton's idea of an atom as too hypothetical. 



His physical atomic theory (described in the fourth 

 paper of this series) was devised in the year 1801, from 

 which time onwards he made many attempts to recommend- 

 it to the scientific world. But for years the only avowed 

 adherent which it obtained was William Henry. The 

 question at issue was a fundamental one, and Dalton's 

 finding on it was ultimately triumphant. The theory ex- 

 pressed his conviction that the diffusion of gases is due to 

 physical forces and not to chemical. But the prevailing 

 tendency of the time was to regard diffusion as due to 

 chemical affinity between the gases concerned, and the 

 strength of that tendency was exhibited by the amount of 

 opposition to Dalton's theory. Its opponents included 



