A common experiment to prove the weight of air is that of the Magdeburg Hemispheres, a simple contrivance of Otto Guericke, a merchant of that city.  It is a part of every complete philosophical apparatus.  It consists of brass caps, which, when joined together, fit tightly and become a globe.  The air within being exhausted, it will be found difficult to separate them.  If the superficies be 100 square inches and the height of the mercury be 30 inches, the atmosphere will press on these hemispheres with a weight of 1,475 lbs, requiring the efforts of seven or eight powerful men to tear them asunder.  One of these instruments, of the diameter of a German ell, required the strength of 24 horses to separate it.  The experiment was publicly made in 1650 at the Imperial Diet at Rendsborg, in the presence of the Emperor Ferdinand III. and a large number of princes and nobles, much to their astonishment.

As compared with water, the air (the barometer indicating 30 deg., and the thermometer 55 deg.) is 833 times lighter.

It is this weight of the atmosphere which counterbalances that of a column of mercury 29 inches in height, and a column of water 32 to 34 feet in height.

The old quaint notion of Nature’s abhorring a vacuum was found to be practically only an assertion that the air had weight.  The ordinary pump, commonly called the suction-pump, is constructed on this principle.  The weight of the atmosphere at the level of the sea is found to be the same all over the world.

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We find the atmosphere with another characteristic,—­Elasticity.

However it may be compressed, air returns, on liberation, to its 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 deg. and 212 deg., 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.