156 Sir H. E. Roscoe and Mr. A. Harden on 



every gas through every other gas, without calling in any 

 other repulsive power than the well-known one of heat. 



" This, then, is the present view which I have of the con- 

 stitution of a mixture of elastic fluids. 



u The different sizes of the particles of elastic fluids under 

 like circumstances of temperature and pressure being once 

 established, it became an object to determine the relative sizes 

 and weights, together with the relative number of atoms in a 

 given volume. This led the way to the combinations of gases, 

 and to the number of atoms entering into such combinations, 

 the particulars of which will be detailed more at large in the 

 sequel.* Other bodies besides elastic fluids, name]y liquids 

 and solids, were subjected to investigation, in consequence of 

 their combining with elastic fluids. Thus a train of investi- 

 gation was laid for determining the number and weight of all 

 chemical elementary principles which enter into any sort of 

 combination one with another." 



As far as quoted, the above narrative is perfectly continuous, 

 so that the statement may fairly be taken as an authoritative 

 one as to the train of reasoning which led Dalton to the 

 attempt to determine the relative weights and sizes of the 

 atoms. 



The only difficulty in the way of regarding the above as 

 a final settlement of the question of the origin of Dalton's 

 theory arises from the date (1805) which Dalton assigns to 

 his ideas about the different sizes of the atoms. The difficulty 

 is this. In Dalton's laboratory note-books there is a table 

 dated September 19th, 1803, in which is given a list of the 

 weights of the ultimate atoms of a number of elements and 

 compounds, together with the specific gravities of such as are 

 gases, and the diameters of their elastic particles, compared 

 with those of water = 1, the very matters mentioned in the 

 concluding paragraph of the foregoing passage. 



The existence of this table shows quite clearly that Dalton 

 had formed definite conceptions regarding the different sizes 

 of atoms as early as 1803, and had not only been confronted 

 with the problem of determining their relative weights and 

 sizes, but had actually obtained numerical values for these, 

 and that, moreover, by the very method which he always 

 afterwards employed. 



These circumstances are in our opinion sufficient to justify 

 us in the belief that Dalton in 1810 by a slip, either of the 

 pen or of his memory, wrote 1805 by mistake instead of 1803. 



The existence of a table such as that just described, drawn 

 * The remainder of the passage is omitted by Debus (p. 355). 



