i68 EXPERIMENTS and OBSERVATIONS 



an unfullied face, would have kept as warm as the air to wind- 

 ward, how much foever the fand might have been colder. 



BUT taking the refult of this experiment as we found it, we 

 may fafely regard it as an inftance of what may be called the 

 fame volume of air in one part, laying down its vapour in the 

 form of hoar-frofl, and in another, paffing away without fuch 

 a decompofition taking place. In the former cafe, we accord- 

 ingly found the correfponding cold produced, but very fmall 

 indications of any fuch thing in the latter. 



As quickfilver thus refifts the hoar-frofl, fo may fome other 

 fubflances ; and, on the contrary, there may be fome much 

 more fitted than others to promote this fort of decompofition of 

 the air in low temperatures. But this fuggefls a variety of ex- 

 periments which I have had no opportunity of purfuing. Cer- 

 tain late appearances have made me think of trying a broad 

 furface, made up of a combination of wool-cards, having ob- 

 ferved fomething like a preference of fharp points in the fettling 

 of hoar-frofl. 



THOUGH it would require many more experiments than our 

 opportunities have allowed of, to underfland fully feveral points 

 belonging to this Enquiry ; fuch as, the fits of refrigeration of 

 the air, and the correfponding aptitude to part with its hoar- 

 frofl, yet the foregoing experiments, regiflers, and obfervations, 

 when attentively confidered and compared with one another, 

 and with thofe narrated in the two papers publifhed in the 

 London Philofophical Tranfaclions, feem to confpire in fupport 

 of the following 



GENERAL CONCLUSIONS: 



That when bodies attraft hoar-frofl; from a clear air, there is a cold 



produced at their furfaces ; and that this cold docs not originate 



from any peculiar qualities of bodies upon -which the hoar-frojl fettles, 



any farther than as fome bodies are capable of attracling from the 



air more or lefs of it in a given time : 



That 



