On Caicarious and Gypjious Earths. 47'^ 



the clouds. Tho' I by no means confider this as a neceflary ' 

 confequencc, fincc therepuHive power of the particles of air, 

 may keep the alkalies and acids they contain without the fphere 

 of each others attraction, yet I am inclined to believe, that 

 this combiaation does aClually take place. To the heat 

 generated by this combination while the falts are forming, I 

 attribute the variations that are felt in the degrees of heat that 

 prevail at different times, in the ftillefl; weather, when fummer 

 heat (hould be uniform ; to the folution of thefe falts, that 

 cold which generates froft and hail in the warmeft feafons. 



There are places among the mountains, and in great forefls, 

 where fro (Is prevail ex.ery month in the year. There are 

 others, which by clearing are freed from this calamity, which is 

 known fo often to diftrefs the firft fetders of a new diftricl. If 

 frofls were occalioned merely by the influence of cold winds, 

 the places mofl expofed to thefe would be fabje6l to them, and 

 woods and yallies would afford a flielter againft: them, a^ indeed 

 they often do againft frofts which are derived from this fource. 

 The reafon of the prevalence of froft in woods and fheltered 

 vallies, when the general temperature of the air elfe where is 

 warm, I fhould take to be the greater exhalations of thofe 

 fubftances that form falts, and the folution of them by the vapours 



Note. The ruft of iron will yield a volatile alkali — nift is scqinred by 

 cxpofure to llie air. [^B/aci.^ 



