239 



tionary, it was found cseteris paribus, that the reduction of pressure 

 arising from cold was at least one-half greater in the anhydrous 

 air, than in the air replete with vapour. This difference seems to 

 be owing to a loan of latent heat made by the contained moisture, 

 or transferred from the apparatus by its intervention, which checks 

 the refrigeration ; yet, ultimately, the whole of the moisture being 

 converted into vapour, the aggregate refrigeration does not differ in 

 the two cases. 



Agreeably to Dalton's tables, at 70° the quantity of moisture in 31 

 grains or 100 cubic inches of air, is yWo °f a g ra i n * The space al- 

 lotted to this weight of vapour being doubled, it would remain uncon- 

 densed at 45° F., being associated with the same weight, but double 

 the volume, of air; but at 32°, notwithstanding the doubling of the 

 space, only -^Uo °f a g ram would remain in the aeriform state; of 

 course 551 — 356 = yoVo"' or near 'y to" °f a grain, would be pre- 

 cipitated. 



The latent heat given out by the condensation of this vapour, 

 would heat, as is well known, 1000 times its weight of water, or 195 

 grains, one degree; or 31 grains X T 9 T 5 = 6.29 degrees; and as the 

 capacity of air for heat is only one-fourth of that of water, it would 

 heat 31 grains of air 6.29 X 4 = 25.16, or nearly 25° F. As air, 

 at 32° F., expands -^-^ for each additional degree, the difference of 

 bulk, arising from the heat received, as above calculated, would be 

 Tim or iV nearly. 



When air, replete with aqueous vapour, was admitted into a re- 

 ceiver partially exhausted, and containing liquid water, a copious pre- 

 cipitation of moisture ensued, and a rise of temperature greater than 

 when perfectly dry air was allowed to enter a vessel containing rare- 

 fied air in the same state. In the instance first mentioned, a portion 

 of vapour rises into the place of that which is withdrawn during the 

 partial exhaustion. Hence when the air, containing its full propor- 

 tion of vapour, enters, there is an excess of vapour which must pre- 

 cipitate, causing a cloud, and an evolution of latent heat from the 

 aqueous particles previously in the aeriform state. Dr. Hare con- 

 ceives that as the enlargement of the space occupied by a sponge, 

 allows, proportionably, a larger quantity of any liquid to enter its 

 cells, so any rarefaction of the air when in contact with water, con- 

 sequent on increase of heat or diminution of pressure, permits a pro- 

 portionably larger volume of vapour to associate itself with a given 

 weight of the air. When, subsequently, by the afflux of wind replete 



