. THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 79 



several extracts from Query 31, and also the enunciation of 

 the 23rd Proposition of Book II of the Prinripia, to which 

 in the 17th lecture he had made a definite reference. From 

 Query 31 Dalton doubtless drew the general conviction that 

 " the Changes of corporeal Things are to be placed only in 

 various Separations and new Associations and Motions of 

 these permanent Particles";* and, in particular, laid hold of 

 Newton's opinion that "God in the Beginning form'd matter 

 in solid, massy, hard, impenetrable, moveable Particles, of such 

 Sizes and Figures, and with such other Properties, and in such 

 Proportion to Space, as most conduced to the End for which he 

 form'd them."f The stress that Dalton lays upon this asser- 

 tion of the possibility of the existence of " particles of matter 

 of several sizes and figures, and in several proportions to the 

 space they occupy, and perhaps of different densities and 

 forces,"J is due to his preoccupation with the problem of 

 accounting for the fact that " the oxygen gas being specifically 

 heaviest should not form a distinct stratum of air at the bottom 

 of the atmosphere and the azotic gas one at the top of the 

 atmosphere."! Before " modern discoveries [had] ascertained 

 that the atmosphere contains three or more elastic fluids, of 

 different specific gravities,"! the difficulty had not arisen. 

 Newton had shown that " particles flying each other with 

 forces that are reciprocally proportional to the distances of 

 their centres, compose -an elastic fluid whose density is as the 

 compression "IF that is which follows a law that Boyle had 

 shewn to be true of ajr; \ Moreover, it was possible to suppose 

 that the repulsion necessary to make the solid atoms of a single 



* Newton, Opticks, p. 376. 



t Opticks, p. 375. 



I This passage is given by Eoscoe and Harden (p. 125) as a further 

 quotation from Query 31. It appears rather to be a note of DaJ ton's own 

 upon the passage printed above. 



Koscoe and Harden, op. tit., p. 14. 



|| P. 13. 



IF Newton's Principia (Motte's trans., 1729), ii, p. 77. 



