i 7 4 EXPERIMENTS and OBSERVATIONS 



hoar-froft to bodies attracting it ? To this it may be anfwered, 

 fince we are confefledly dealing in furmifes and conjectures, 

 that, poffibly, in the mixed fubject we are now contemplating, 

 there may be a third ingredient, of an electrical or phlogiftic 

 kind, and which is neceflarily feparated from the air along with 

 the hoar-froft. 



ACCORDING to this view, with the help of Dr CRAWFORD'S 

 general fact mentioned above, we could account for the cooling 

 procefs, by faying, that it depends upon the incrcafed capacity 

 of the air for heat, in confequence of its being dephlogifticated 

 by the mediation of a third fubftance, namely, the hoar-froft, 

 which, by a predominant affinity, in certain cafes, is made to 

 part from the air, and unite itfelf to the attracting fubftance. 



IT would require, however, many nice and well ordered ex- 

 periments, before the merits of a fuggeftion, fo arbitrary as this 

 is, could be determined. 



BUT how much foever we may be at a lofs to underftand in 

 what manner cold is produced by the feparation of hoar-froft 

 from a clear air, or what becomes of the xenfible heat which, 

 in that procefs, conftantly difappears, yet, by regarding the 

 thing itfelf as a general fact, now demonftrated by a competent 

 induction, it feems already to offer us fome afliftance in explain- 

 ing, by the fynthetical method, certain phenomena connected 

 with our fubject. Of this, what has been mentioned of the 

 two thermometers, page 172. may ferve as an example. 



IN the firft cafe, we know fo much of the nature of evapo- 

 ration as to be certain, that any portion of air, relatively dry, 

 at meeting the finall mafs of fnow furrounding the ball, muft 

 produce cold, more or lefs, even by the moft tranflent contact. 

 By a conftant fucceffion, therefore, of fuch evaporating air, the 

 mercury in the thermometer will be made to fink very fenfibly 

 below that of another expofed to the fame current, having no 

 mow round its ball. 



BUT 



