2 Proceedings of the American Philosophical Society. 45 
as when the air was not in contact with liquid water; being nearly as 
nine to five. 
Under the circumstances last mentioned, the hygrometer was motion- _ 
less; whereas, when no liquid water was accessible, the space, although 
previously saturated with vapor, by the removal of a portion of it together 
with the air which is withdrawn by the exhaustion, acquires a capacity 
for more vapor ; and hence the hygrometer, by an abstraction of one 
third of the air, revolved more than sixty degrees towards dryness. But 
when a smaller receiver (after being subjected to a diminution of pressure 
of about ten inches of mercury, so as to cause the index of the hygrome- 
ter to move about thirty five degrees towards dryness) was surrounded by 
a freezing mixture, until a thermometer in the axis of the receiver stood 
at three degrees below freezing, the hygrometer revolved towards damp- 
hess until it went about ten degrees beyond the point at which it rested 
when the process commenced. 
It appears, therefore, that the dryness produced by the degree of rare- 
faction employed is more than counterbalanced by a freezing temperature. 
As respects the heat imparted to the air above mentioned, the fact, that 
the ultimate refrigeration in the case of air replete with vapor, and in that 
of anhydrous air, was equally great, and that when water was present 
cold was greater in the damp vessel, led to the idea that the heat 
arising under such circumstances could not have much efficacy in aug- 
menting the buoyancy of an ascending column of air: but when, by an 
appropriate mechanism, the refrigeration was measured by the difference 
of pressure at the moment when the exhaustion was arrested, and when 
the thermometer had become stationary, it was found ceteris 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 vapor. This 
difference Seems to be owing to a loan of latent heat made by the con- 
tained moisture, or transferred from the apparatus by its intervention, 
Which checks the refrigeration ; yet, ultimately, the whole of the mois- 
ture being converted into vapor, 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 °2;'; of a grain. The space allot- 
ted to this Weight of vapor being doubled, it would remain uncondensed 
at 45° -, being associated with the same weight but double the volume 
of air; but at 32°, notwithstanding the doubling of the space, only 74ys'5 
a grain would remain in the aériform state; of course = 
Toes, OF nearly 2; of a grain would be precipitated. 
te latent heat given out by the condensation of this vapor, would 
at, as 
— — ‘YP =6.29 degrees ; and as the capacity of o = 
rth of that of water, it would heat 31 grains of air 
is well known, 1000 times its weight of water, or 195 grains, one . 
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+ 
