l6 yiKLD'RXl'M, Deve/opjnent of the A to)inc TJicory. 



" 2. The ult. atoms are of the same relative weight as 

 the gases themselves. 



" 3. That neither of these positions is accurate. 



" According to the first the gases of greatest specific 

 gravity are those whose particles are closest and the 

 diameters of the elastic particles will be as the cube root 

 of the sp. gr. This cannot be true for nit. gas which is 

 made up of azot and oxygen is lighter than oxygen 

 itself ; and so is aq. vapour than oxygen one of its 

 •constituents." ^^ 



" According to the 2nd position all gases will have 

 the same number of particles, and consequently the same 

 distances of each in a given volume, under like circum- 

 stances. This position is contradicted by facts : for all 

 ■compounds would be heavier than their simples upon this 

 principle, which is contrary to experience. 



" The two former positions being disproved, it follows 

 that when two gases of like force, &c., are presented to 

 each other, the number of particles in a given surface of 

 one of them will not be the same as in the other ; conse- 

 quently, no proper equilibrium can take place." -" 



This material is as important as anything on the 

 subject can well be. The pages quoted, Nos. 109 and 

 III of the note-book, amount to a summary of Dalton's 

 reasoning on the subject of the sizes of atoms, leading to 

 his decision in favour of the new diffusion hypothesis. It 

 is easy to assign a date to this decision. By reason of the 

 subject-matter, pages 107, 109, and iii are closely con- 

 nected with one another. Page 107 contains a table of 

 the weights and diameters of atoms, a table which, it may 

 well be supposed, was drawn up in order to illustrate 

 X)alton's inquiry into the sizes of atoms. It is dated 



-^ Note-books, vol. i, p. 109. 

 '^' Op. cit., p. III. 



