relative to Black, Watt, and Cavendish. 511 



observation, that in very cold frosty weather fires burn most 

 briskly*." 



But we are by no means to conclude from hence that Hales 

 had any doubt of the plurality of elastic fluids; on that point 

 he quotes, as at once the foundation and the result of all his 

 inquiries, the opinions expressed by Newton in the Optics : — 

 "The illustrious Sir I. Newton," he begins, "observes, that 

 true permanent air arises by fermentation, or heat, from those 

 bodies which chemists call fixed, whose particles adhere by a 

 strong attraction, and are therefore not separated or rarefied 

 without fermentation, those particles receding from one an- 

 other with the greatest repulsive force, and being most dif- 

 ficultly brought together which upon contact are most strongly 

 united." " Dense bodies by fermentation rarefy into several 

 sorts of air, and this air by fermentation, and sometimes with- 

 out it, returns into dense bodiesf," " of the truth of which," 

 Hales adds, " we have proof from many of the following ex- 

 periments." And as he begins, so he ends : for having again 

 repeated the same quotation from Newton, he closes his " ana- 

 lysis of air " by drawing this general inference from all his 

 researches — " Since we find in fact from these experiments 

 that air arises from a great variety of dense bodies both by fire 

 and fermentation, it is probable they may have very different 

 degrees of elasticity in proportion to the different size and 

 density of their particles, and the different forces with which 

 they were thrown off into an elastic state." 



What now, give me leave to ask, becomes of your state- 

 ment, that "when D'Alembert wrote the article l Air' in the 

 Encyclopedic in 1751, he gave the doctrine then universally 

 received, that all the other kinds of air were only impure at- 

 mospheric air, and that this fluid alone was permanently elas- 

 tic?" You tell us elsewhere that D'Alembert disregarded 

 inductive philosophy, and professed himself ignorant of che- 

 mistry : and thus I should have accounted for his ignorance 

 on this point, if I had not found on consulting the volume 

 which you quote, that he really expressed no such opinion 

 respecting air, and moreover has stated fully the views enter- 

 tained of it by those, who in his own words, "supposent qu'il 

 peut etre produit et engendre, et que ce n'est autre chose que 

 la matiere des autres corps, devenue par les changemens qui 

 s'y sont faits, susceptibles d'une elasticite permanente." D'A- 

 lembert says indeed, that some of the ancients considered the 

 air as a simple element, but remarks with truth, that they did 

 not attach the same sense to that term as ourselves. 



I have now completed the sketch which I promised, of the 

 * Analysis of the Air, edit. 1727, p. 247. t Ibid. p. 312. 



