118 Journal of the Mitchell Society [Jan. 



pure elastic fluid presents to the imagination a picture like one 

 full of small shot. The globules are all of the same size, but the 

 particles of the fluid differ from those of the shot in that they 

 are constituted of an exceedingly small central atom of solid 

 matter, which is surrounded by an atmosphere of heat, of great 

 density next the atoms but gradually growing rarer according to 

 some power of the distance: whereas those of the shot are glo- 

 bules uniformly hard throughout and surrounded with atmos- 

 pheres of heat of no comparative magnitude." 



How stubbornly the notion of the material nature of heat 

 was held to may be realized from an examination of Lavoi- 

 sier's explanation of the expansion and contraction of matter 

 and from the fact that it was included by some in the list of 

 elements for years after the time of Dalton. 



According to Dalton's hypothesis these atoms of definite and 

 distinctive weight combined under certain fundamental laws 

 to form the various substances that make up the material world. 

 As to the size and form, the chemist gave himself no concern, 

 but took up the task of finding out the exact, relative weight of 

 every different atom known to him. These atoms he believed to 

 be unchangeable and thus made possible a mathematical basis 

 for his science. 



But the physicist is necessarily concerned with the atom also, 

 and at the same time that Dalton reaffirmed the atomic theory. 

 Young gave his reasons for regarding the atom as a perfect 

 elastic sphere in the place of the conceptions that had grown up 

 of mathematical points, various other geometric forms and 

 rigid particles. 



The Daltonian atomic theory was slow of acceptance because 

 of the confusion arising between the divisible ultimate particles 

 of compounds and those which so far as was known were unde- 

 composable. A clear distinction had to be drawn between atom 

 and molecule and this was not easy until many facts had been 

 accumulated. Chemists came to know many different kinds of 

 atoms representing as many different elements. These elements 

 were at first called simple bodies, but to accurately define an 



