Dalton's Atomic Theory. 365 



far as I know, never taken exception to any of its statements. 

 Now Thomson always calls the relative weight of atoms the 

 " density of atoms." His table of molecular weights is called 

 table of the density of atoms (molecules) (vol. iii. p. 429). 

 Elementary physics teaches that the weights and densities of 

 substances are proportional when the substances under con- 

 sideration are of the same volume. If we say for density 

 specific gravity, then Thomson's report of Dalton's atomic 

 views, and Dalton's statement in Note-book, i. p. 260 (R. 42) 

 are in perfect agreement. Accordingly, Dalton had not, in 

 August 1804, thrown the hypothesis M/S = C overboard. He 

 did so in the following year, 1805, when he could not bring it 

 into agreement with his atomistic views (p. 351) ; and when 

 he found it refractory in this respect, he called it " a confused 

 idea" (N.S. 188). 



Messrs. Roscoe aud Harden charge me on pp. 10-11 of 

 their book with a " confusion of ideas." They say : — " Debus 

 further argues that when Dalton communicated his theory to 

 Thomson, he must have held the opinion that these two 

 relations, the relative density and the relative weight of the 

 atoms, were identical, or what Debus appears to consider to 

 be the same thing, that the relative densities of the gases 

 were identical with the relative weight of their atoms. . . . 

 On the strength of this argument, which appears to rest 

 on a confusion between the relative density of the atoms and 

 the relative density of the gases made up of those atoms. . . " 

 1 have not asserted that the relative weight and the relative 

 density of the atoms were identical. I have expressed my- 

 self in the following manner : — " We must conclude that 

 Dalton, at the time when he communicated his ideas to 

 Thomson, considered that the molecular weights of the gases 

 stand to each other in the same ratio as the densities of their 

 molecules " (D. 51) . As for the confusion between the relative 

 density of the atoms or molecules and the relative density of 

 the gases made up of those atoms or molecules, I must refer 

 to p. 361, where it has been shown, if we write for density 

 specific gravity, that according to Dalton's own definition, 

 the specific gravity of a gas is equal to the specific gravity of 

 one of its molecules. Therefore, the confusion is not on my 

 side ! 



The Italian physicist Amadeo Avogadro became acquainted 

 with Dalton's atomic theory through Thomson's report (0. 

 no. 8, p. 7, note). He puts on Thomson's statements the 

 same interpretation as I do. Dalton's object is to find the 

 atomic weights of the elements, and the molecular weights of 



