165 
MAGAZINE'OP SCIENCE AND ART. 
ther than before the elastic force with which it 
presses against the sides of the vessel increases. If 
now an opening be made in the vessel, air will rush 
out until its elastic force has been reduced to its 
former natural amount; but it is plain that the 
specific gravity of the air has diminished, since, 
while the total space remains the same, the weight 
of air within it has been decreased by the quantity 
which has rushed out. Such are the simple, but 
often perplexing, relations of volume or space, 
elastic force or pressure, and specific gravity or 
density, as affected by gravity and by heat. 
13. But we must consider not only the nature of 
the effects of heat, but also the degree, quantita¬ 
tively, in which any given amount of caloric or 
heat will produce these effects. Each substance 
has a particular capacity, or so to speak, an appe¬ 
tite for heat, called, in scientific language, its 
specific heat. These capacities for heat express the 
relative quantities of caloric or heating power, 
which are required to produce equal apparent effects 
in different substances, as measured by the ther¬ 
mometer. 
Now, the specific heat of air is not always the 
same, it depends entirely upon the rarity or den¬ 
sity of the air, and increases in some ratio as the 
density decreases. If we take two equal weights of 
air, and enclosing the one in a vessel of one cubic 
foot of space, allow the other to expand into a ves¬ 
sel of, say, twice the size, and if we then apply a 
uniform source of heat to both, it would be found, 
supposing a sufficient delicacy attainable in so 
rough an refinement, that the vessel in which the 
air was most expanded (viz., the two cubic-foot 
vessel) would take a longer time than the other, to 
rise through 10 degrees of Fahrenheit's scale for 
instance. Dense air has less appetite for, or ab¬ 
sorbs less heat than rarefied air, but the ratio in 
which the specific heat thus increases with the 
rarety is so difficult to determine experimentally, 
that I believe it has never yet been satisfactorily 
accomplished, or perhaps scarcely attempted. 
14. But what will be the effect, now, of this vari¬ 
ation of the specific heat, if, instead of applying 
heat or cold, we merely vary the density of a body 
of air, by applying or withdrawing pressure. When 
the air is compressed, its capacity for heat is dimin¬ 
ished; it can no longer contain as much as before, 
and a portion of heat may be actually said to be 
squeezed out. What was before latent heat, becomes 
sensible heat, and causes a rise of temperature, or 
intensity of heat, as measured by the thermometer. 
Air, in fact, closely resembles a moist sponge, 
which may look as if almost dry, until, on being 
pressed, water appears at every part, and perhaps 
runs off it. The moisture which was latent, or 
hidden in the sponge, becomes sensible, when the 
absorptiveness is decreased. 
But what, on the other hand, will he the effect 
upon air if we withdraw pressure, and allow it to 
expand from its own elastic force ? Obviously the 
reverse. Its capacity for heat will increase, and 
the same apparent temperature cannot be main¬ 
tained, because it would, therefore, indicate an in¬ 
creased total amount of caloric, a supply of which 
is not forthcoming. The sensible temperature 
must, therefore, fall, and heat will appear to be ab¬ 
sorbed, or rendered latent, just as moisture is again 
absorbed when pressure is withdrawn from the 
sponge. The one becoming apparently dry, is ana¬ 
logous to the other becoming apparently cold. 
15. We can now understand the interference 
which the variation of specific heat occasions in 
the atmosphere, which, as I have said, uniformly 
diminishes in density the higher we ascend into it. 
A cubic foot of air, if gradually raised from the 
surface, would as gradually tend to increase in vo¬ 
lume, from the diminution of pressure, until on 
arriving at the upper limit of the atmosphere, it 
would expand into an almost indefinite space. Its 
specific heat would, consequently, rise in a very 
great ratio, heat would he absorbed in large quan¬ 
tities, and the sensible temperature fall gradually, 
but to an almost indefinite extent. It is commonly 
said, that the temperature decreases in the atmos¬ 
phere at the rate of one degree Fahrenheit for every 
100 yards of ascent; but this rule probably does 
not hold true to any great height. As it is clear, 
that the variation of temperature must affect the 
otherwise simple relation of specific gravity to elas¬ 
tic force, it will be seen that the gradation of all 
the properties of the atmosphere are mutually de¬ 
pendent in so complicated a manner, as almost to 
defy the solution of the problem. Philosophers 
are even now divided, like two parties of politi¬ 
cians, upon the rather fundamental point, whether 
the atmosphere is definitely terminated above, or 
indefinitely extended. 
16. The similitude of a moist sponge may be, 
with advantage, further dwelt upon. It is usual to 
liken the atmosphere to a huge pile of cotton-wool, 
of which the higher portions, though light and 
flocky, compress the lower parts, by their accumu¬ 
lated weight, into dense layers. If we can imagine 
a vast heap of moist sponges, we shall have a rough 
analogy, not only to the gradation of pressure, but 
also to the gradation of temperature, for the lower 
layers of sponge, strongly compressed by the super¬ 
incumbent weight, will exhibit much apparent 
moistness, just as the lower parts of the atmosphere 
exhibit a greater apparent temperature. The in¬ 
compressible nature of water would retard the com¬ 
pression in the lower layers of sponge, just as the 
increased sensible temperature must tend to do in 
the air. 
17. Having now explained the relation of heat 
to air, we must consider the even more complex 
effects which heat produces in the condition of 
water. 
Whatever the temperature of water, or of ice, it 
always exhibits a tendency to expand into the 
gaseous form, or to give off aqueous vapour. This 
expansive tendency, or elastic force, is, however, of 
an exactly limited amount; at the freezing point it 
is just equal to two-tenths of an inch depth of mer¬ 
cury,-or to the l-150th part of the average pressure 
of tho atmosphere at the surface of the earth. It 
increases, however, very rapidly with the tempera¬ 
ture, in fact, at a very much quicker ratio than the 
latter, so that 212 degrees, the elastic force of 
vapour, is equal to 30 inches of mercury, or to the 
whole pressure of the atmosphere, and the water is 
on the point of boiling. 
18. There is a point of the subject here which it 
is of the utmost importance to understand com¬ 
pletely, although it is by no means easy to explain 
it clearly, namely, that (he elastic force with which 
the surface of liqnid or solid water tends to throw 
off vapour,, is quite distinct from the clastic force 
which that vapour, after once attaining the gaseous 
condition, and becoming removed from the contact 
of water, may exert at various temperatures. If we 
take a cubic foot of pure aqueous vapour, and en¬ 
closing it in a dry vessel of that capacity, apply 
