original volume, and while thus perfectly elastic it is also the most
compressible of bodies. This elasticity arises from the repulsive
force of its particles, and is always equal to the compressive force
which it balances. A glass vessel full of air, placed under a receiver
and then exhausted by the air-pump, will burst into atoms. Water, on
the other hand, is almost the reverse. Twenty cubic inches, introduced
into a cannon whose sides are three inches thick, cannot be compressed
into nineteen inches without bursting it. This non-elastic property of
water, with another, that of communicating, when under the action of
any force, an equal pressure in all directions, led to the invention
of the hydraulic press.

The elasticity of the air enables fishes to rise and sink in water,
through the action of the air-bladder.

The sudden compression of air liberates its latent heat, and produces
fire. On this principle the pneumatic tinder-box is constructed.

Brockhaus says that air has as yet been compressed only into
one-eighth of its original bulk.

For every degree of heat between the freezing-point and the boiling-point,
32° and 212°, the expansion of air is about 1/490th part, so that
any invention which seeks to use rarefied air as a motive power
must employ a very intense degree of heat, enough to fuse many kinds
of metals.

To the celebrated Mr. Boyle and to Henry Cavendish, both of Great
Britain, we are indebted for most of what we know of this particular
property of the air.