Dalton' s Atomic Theory. 357 



Dalton," that the sizes of the particles of gases must be dif- 

 ferent. For a measure of azotic gas and one of oxygen, if 

 chemically united, would make nearly two measures of nitric 

 oxide, and those two could not have more atoms (molecules) 

 of nitric oxide than one measure had of azote or oxygen. 

 Hence the suggestion that all gases of different kinds have a 

 difference in the size of their molecules ; and thus we arrive 

 at the reason for that diffusion of every gas through every 

 other gas . . . . " (R. 16-17). The old view of Dalton, that 

 in gases the force of cohesion is quite overcome by the force 

 of repulsion, and that, in consequence, the particles of the 

 elementary gases consist of single atoms, would lead to the 

 conclusion which he drew from the volumetric proportions of 

 nitrogen and oxygen in nitric oxide, without requiring the 

 atomic theory as described on p. 353. 



Passing on to the atomic theory we have the remarks : — 

 " The different sizes of the particles of gases, under like 

 circumstances of temperature and pressure, being once estab- 

 lished, it became an object to determine the relative sizes and 

 weights together with the relative number of atoms in a given 

 volume." 



If we remember that these quotations are not from a running 

 narrative, but from notes intended for a lecture, where the 

 lecturer could at any moment make verbal explanations and 

 additions, then we may ask whether, between the notes on the 

 diffusion of gases and those on the atomic theory, there is, or 

 there is not, a connexion similar to the one between cause and 

 effect, as Roscoe and Harden assume ? On this point Dalton is 

 absolutely silent. We wish to know the origin of the methods 

 by means of which Dalton determined the size, weight, and 

 numbers of the atoms ; and we cannot discern how he arrived 

 at these methods from his theory of gaseous diffusion. Messrs. 

 Roscoe and Harden do not say one word on this subject. May 

 not Dalton have been engaged in the calculation of molecular 

 and atomic weights and molecular volumes before he recog- 

 nized that the size of the molecules of different gases must be 

 different ? A few facts show that he was so engaged. 



The idea of considering the influence of the size of the mole- 

 cules on diffusion occurred to Dalton, according to his own 

 statement, in 1805. The result of his consideration was the 

 abandonment of the theory of diffusion formed in 1801 (de- 

 scribed on pp. 354, 355), and the formation of a new view, 

 which ascribed diffusion to difference of size of the molecules 

 of the diffusing gases. 



This new view is, according to Roscoe and Harden, the 

 source of Dalton's atomic theory. If so, then the atomic 



