158 Sir H. E. Roscoo and Mr. A. Harden on 



on p. 246 of the Note-book, i., and is as follows (R. and H. 



p- 27 ) : -. . . . . 



" Enquiry into the specific gravity of the ultimate particles 

 or elements. 



Though it is probable that the specific gravities (sic) of 

 different elastic fluids has some relation to that of their ulti- 

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

 thing ; for the ult. part, of water or steam are certainly of 

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

 is heavier than steam. " 



The second passage (also 1803) occurs on p. 260 (R. and 

 H. p. 42) in the form of a heading to a table which runs : 

 " Ultimate atoms of gases in order of their specific gravities.''' 

 Then follow numbers which represent the relative iveights of 

 the various atoms. 



Finally, Thomson, in his account of the atomic theory, 

 consistently uses the expression density or relative density of 

 the atom where we should rather expect him to use the term 

 relative weight. 



Debus professes not to be able to understand the first of 

 these passages, and argues from the others that if the specific 

 gravities or relative densities of the atoms are identical with 

 their relative weights, it follows that their volumes must all 

 be equal, and hence Dalton must have retained his belief in 

 the theory M/S = C until at least 1804, since it was not 

 until that year that he communicated the atomic theory to 

 Thomson. 



It will at once be seen that in these passages Dalton uses 

 the term " specific gravity " as applied to an atom^ with the 

 meaning of " relative weight." The passages when thus in- 

 terpreted run as follows : — " Enquiry into the relative weights 

 of the ultimate particles or elements. 



" Though it is probable that the specific gravities of dif- 

 ferent elastic fluids have some relation to the relative weights 

 of their ultimate particles, yet it is certain that they are not 

 the same thing ; for the ult. part, of water or steam are 

 certainly of greater relative weight than those of oxygen, yet 

 the last gas is (specifically) heavier than steam. " 



The second passage becomes : — " Ultimate atoms of gases 

 in the order of their relative weights. " 



In this form both extracts are quite comprehensible and con- 

 sistent, both with each other and with Dalton's other expres- 

 sions. It was in this way that we at first interpreted them 

 (R. and H. pp. 27, 43), and Debus himself (p. 362) makes 

 the same suggestion for the first passage. He then, however, 

 takes this amended passage, contrasts it with the second in 



