Dalton's Atomic Theory. 361 



Dalton, in his Note-book, i. p. 246 (R. 27), has the following 

 memorandum : — 



" Though it is probable that the specific gravities of different 

 elastic fluids (gases) has some relation to that of their ultimate 

 particles, yet it is certain that they are not the same thing ; 

 for the ulterior particles of water or steam are certainly 

 of greater specific gravity than those of oxygen, yet the last 

 is heavier than steam/ 3 



Instead of building on the dictum contained in this quotation 

 weighty conclusions, as Messrs. Roscoe and Harden do, I 

 confess that I do not understand it. What is the difference 

 between the specific gravity of a gas and that of one of its 

 ulterior particles or molecules ? According to Dalton, the 

 molecular volume is =M/S (JST.S. 226, note). The specific 

 gravity of a molecule, s, is expressed by the quotient of the 

 molecular weight and molecular volume : 



Molecular weight _ M _ ~ _ 



Molecular volume M/S 



Hence, S, the specific gravity of the gas, = s, the specific 

 gravity of one of its molecules ; or, in other words, both are 

 the same thing. This result follows from Dalton's definition 

 of the molecular volume, and is opposed to his assertion that 

 the specific gravities of gases and the specific gravities of their 

 molecules are not the same thing, and also to the conclusions 

 of Roscoe and Harden, based on this erroneous assertion. 

 Therefore it does not follow that Dalton did never believe in the 

 hypothesis M/S = C, or that the first part of my view stated on 

 p. 360 is incorrect. 



Messrs. Roscoe and Harden assert that he (Dalton) " never 

 appears to have believed in the law of equal volumes (hypo- 

 thesis M/S = C), and this only occurred to him 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 gases agree in the size of their particles " (R. 47). 



What does Dalton himself say on this subject? — " At the 

 time I formed the theory of mixed gases (year 1801) I had a 

 confused idea, as many have, I suppose, at this time, that the 

 particles of gases are all of the same size, that a given volume 

 of oxygen contains just as many particles as the same volume 

 of hydrogen " (N.S. 188). And with regard to the statement 

 " that no two gases agree in the size of their particles," he 

 says, " This idea occurred to me in 1805/' Therefore the 

 idea that a given volume of oxygen contains just as many 

 particles as the same volume of hydrogen occurred to Dalton 

 at least four years before the time at which he recognized 



