$7. 7 Rt 
" Quantity of Heat evolved from Atmospheric Air. 43 
, ‘rarefied until its density was three-fifths of its natural density, 
. when suddenly restored acquired about 48° F’, of temperature. 
Professor Espy states as the result of experiment, that if air 
have: its density doubled or be reduced to half its volume, its 
temperature will be increased about 90° F., and that air expanded 
to half its density, or double its volume, generates a cold of about 
90° F. 
That accurate and profound mathematician, the late Mr. Ivary, 
has attempted to deduce from calculation, upon data to which I 
have been unable to get access, the precise quantity of caloric 
given out by the condensation of atmospheric air. According to 
his estimate one degree of heat is evolved from it, when under a 
condensation equal to ;1, of its volume; and consequently, if a 
volume of air be reduced to half its bulk the heat given out will 
be equal to 180° F. Both Laplace and Leslie, in attempts to 
reconcile the calculated velocity of sound, with that which is de- 
duced from experiment, made similar calculations; but the theo- 
rems by which they are expressed are both abstruse and inaccu- 
rate, for though they regard the quantity of heat evolved from air 
by condensation as “ profuse and powerful,” the amount the lat- 
ter assigns is less than that mentioned by Mr. Ivary; and his 
estimate, it will be seen, does not equal the truth. 
e researches of M. Gay Lussac, to whom science is more 
indebted for an extensive and profound investigation of this sub- 
ject than to any other philosopher, led him to consider a conden- 
sation of air into one-fifth of its volume as sufficient to ignite 
tinder—a degree of heat which he states is almost 580° F. This 
estimate, applied to volume, according to what has been mentioned 
as the custom of experimenters, would give about 280° EF’. for 
every time the density of air was doubled. 
pon a subject on which so many scientific men have directed 
their attention and tasked their ingenuity to discover methods of 
accurate observation, it would seem presumptuous to attempt to 
devise processes more exact than have been already employed. 
Yet the diversity in the results furnished by such able experi- 
-Menters renders questionable the truth of any of them, and shows 
that their modes were defective, or, at least, that fresh investiga- 
tion is necessary to enable us to confirm or reject their data with 
certainty. That in the general economy of nature the relation 
of caloric to air is of high importance, and that the investigation 
of the laws of their action is worth the labor they may require, 
and is indeed one of the most interesting enquiries, in both a sci- 
entific and practical point of view, to which the attention of 
Mankind can be turned, are apparent from considerations already, 
alluded to, 
To improve our knowledge of these relations, so as to deduce 
laws by which they are governed, we must study the subject 
