Upon a REMARKABLE COLD. 15,9 



moifture, depend much upon its ftate of electricity. It might, 

 therefore, be much worth while to examine experimentally, how 

 far the fame principles may co-operate with the more general 

 caufes above affigned, in regulating the affinity betwixt the air 

 and the hoar-froft difTolved in it ', and how far the like pheno- 

 menon of cold may take place upon the feparation of night- 

 dew from a clear air in milder feafons. 



IF the watery principle, when united to the air in a warm as- 

 well as in a very cold temperature, exifted in the ftate of vapour, 

 or under any other form and conftitution fimilar in both cafes, 

 one fhould expect fome difference in the phenomena, when, in 

 the firft cafe, it is yielded to bodies in the form of moiflure, 

 and, in the lafl, in the form of hoar-froft ; becaufe we are cer- 

 tain, from Dr BLACK'S difcoveries, that the fame quantity of 

 the fame kind of matter under the one and the other of thefe 

 two forms, contains a very different quantity 4 of abfolute heat. 



UPON the prefent fuppofition, therefore, if it mould be found, 

 that the cold produced when bodies attract moifture from the 

 air, is no greater than what is produced when they attract hoar- 

 froft, this would amount to a proof, that the cooling procefs is 

 much more active in the laft cafe than in the firft ; or, in other 

 words, that the wafte of fenfible heat, effected by the unknown 

 caufe, is much greater, in the act of feparation, when the aic 

 depofits hoar-froft, than when it depofits moifture. 



SECT. II. 



HAVING now been affured, by fo many arguments drawn 

 from experiment, that the excefs of cold at the fnow had a con- 

 ftant dependence upon its attracting hoar-froft from the air,. I 

 was next defirous of learning, whether an equal degree of cold 

 would obtain when the hoar-froft was attracted by fubftances 

 of a fimilar loofe contexture. 



THE 



