362 Dr. H. Debus on the Genesis of 



" that no two gases agree in the size of their particles." Con- 

 sequently, the first idea (M/S = C) did not " occur to him only 

 as a possible alternative, at once shown to be inconsistent with 

 fact, to the statement which he recognized as the true one, viz. 

 that no two elastic fluids agree in the size of their particles." 

 but was held by Dalton, more or less, from 1^01 to 1805, and 

 only abandoned when he could not bring it into agreement 

 with his later atomistic views. 



It has been shown on p. 361 that the memorandum in Dalton's 

 Note-book, i. p. 246, is not correct, and that the specific gravity 

 of a gas and the specific gravity of one of its molecules are the 

 same thing. But how, we may ask, could Dalton contradict 

 his own definition? On p. 260 of his Note-book, i. (R. 42), 

 occurs a table with the following heading : — " The molecules 

 of gases arranged according to their specific gravities;" and 

 then follow in arithmetical order the molecular weights of 

 eleven gases. The specific gravity of the molecules and the 

 molecular weights are therefore expressed by the same 

 number. 



If M represents the molecular weight, s the specific gravity, 

 and v the volume of a molecule, we have 



M/v = s. 



As M is put — s, v must be =1; and as this relation is 

 assumed to be general, all gases have the same molecular 

 volume, or M/S = C. Hence the molecular weight and the 

 specific gravity of a molecule are then, according to the table, 

 " the same thing/'' 



If we now substitute in Dalton's memorandum quoted on 

 p. 361, which is not intelligible by itself, for the words " spe- 

 cific gravities of molecules" the words "molecular weights/' 

 we obtain : — " Though it is probable that the specific gravities 

 of different elastic fluids (gases) has some relation to their 

 molecular weights, yet it is certain that they are not the same 

 thing " (R. 27) — a perfectly intelligible statement. 



Therefore, on p. 246 of Note-book, i., the hypothesis M/S = C 

 is rejected, and on p. 260 accepted. 



How are these contradictory statements to- be explained ? 



On p. 188 of his celebrated work, ' New System of Che- 

 mical Philosophy,' Dalton says : — " At the time I formed the 

 theory of mixed gases, I had a confused idea, as many have, 

 I suppose, at this time, that the particles of gases are all of the 

 same size." Consequently there was a time when Dalton 

 believed in the hypothesis M/S = C, and his endeavour was 

 to verify the same by experiment. Accordingly, he lost no 

 time in calculating M for oxygen and steam, as soon as he had 



