Pee eS, ere, eee pth ap aa See Cee Ee ey CS re Br eee Ie Peg eT PR Ne ie ae ee NS ee Pe he a ee eS ye ee eee 
_ pands, and in so doing absorbs so much caloric as to precipitate 
_ contains in a shower of very white compact snow, or rather hail, which may 
ark held in the ”— Sillimai 
B. V. Marsh on the Luminosity of Meteors. 93 
that its volume can not be increased, the amount of heat re- 
quired to produce the change of temperature is called ‘the spe- 
cific heat under a constant volume.’”—Silliman’s Physics, p. 459. 
The specific heat of air under constant pressure (that of water 
being unity) has been found to be 0:2877; specific heat of air 
under constant volume has been found to be 0°1678; difference 
0-0699. 
“Comparing these results in the case of air, we see that, when 
air is heated in a situation where it is free to expand, only about 
§ of the heat applied is expended in producing elevation of tem- 
perature—as in heating a room—while about 2 of the heat is 
expended in producing expansion of the air, to be given out 
again as the room cools,”—wSilliman’s Physics, p. 451. 
Again: “It is a perfectly well known fact that a certain 
amount of heat is rendered latent in producing the expansion 
of a given mass of gas, and that, on condensing the gas to its 
original volume, the same amount of heat is set free.” —Cooke’s 
Chemical Physics, p. 480. 
The absorption of & certain amount of heat, and the rendering 
of it latent, appears to be admitted as a necessary accompani- 
ment of the act of expansion, as such, and essential to its accom- 
plishment—whether the expansion be produced by the removal 
of pressure,’ or by the application of heat, or by both combined. 
The amount absorbed must therefore depend solely upon the 
extent of the expansion—and air of any given density must 
always contain the same amount of latent heat, no matter what 
may be its past history or its present condition as to temperature 
when air is heated in a situation where it is free to expand with- 
_ out change of pressure, equal additions of heat make equal addi- 
nds ;1, part of its bulk for every 
expa r¥ s bu 
_ added degree of heat on Fahrenheit’s scale. That is 
491 cubic inches of air at 32° become 
a02. Ho ee gg 
493 .* ee - 34 " &e, 
A striking instance of the effect of the removal of pressure is afforded on 4 
2 4 
_ Yast scale at the fountain of Hiero, at the mines of Chemnitz in Hungary, “A part 
i lar column 
__ of the machinery for working these mines is a of water 260 
_ feet high, which presses on a quantity of air in a ir. The air 
Is consequently condensed to an enormous degree by this height of water, which is 
- equal to 8 or 9 atmospheres; and when a pipe communicating with the reservoir of 
air is suddenly opened, it rushes out with extreme velocity, instantly ex- 
te the moisture it 
readily gathered in a hat blast. 
