[Illustration:  FIGS. 54, 55.—­As the pressure on the gas increases, its volume decreases.]

Pour more mercury into the open end of the tube, and as the mercury rises higher and higher in the long arm, note carefully the decrease in the volume of the air in the short arm.  Pour mercury into the tube until the difference in level bd is just equal to the barometric height, approximately 32 inches.  The pressure of the air in the closed end now supports the pressure of one atmosphere, and in addition, a column of mercury equal to another atmosphere.  If now the air column in the closed end is measured, its volume will be only one half of its former volume.  By doubling the pressure we have reduced the volume one half.  Similarly, if the pressure is increased threefold, the volume will be reduced to one third of the original volume.

90.  Heat due to Compression.  We saw in Section 89 that whenever the pressure exerted upon a gas is increased, the volume of the gas is decreased; and that whenever the pressure upon a gas is decreased, the volume of the gas is increased.  If the pressure is changed very slowly, the change in the temperature of the gas is imperceptible; if, however, the pressure is removed suddenly, the temperature falls rapidly, or if the pressure is applied suddenly, the temperature rises rapidly.  When bicycle tires are being inflated, the pump becomes hot because of the compression of the air.

The amount of heat resulting from compression is surprisingly large; for example, if a mass of gas at 0 deg.  C. is suddenly compressed to one half its original volume, its temperature rises 87 deg.  C.

91.  Cooling by Expansion.  If a gas expands suddenly, its temperature falls; for example, if a mass of gas at 87 deg.  C. is allowed to expand rapidly to twice its original volume, its temperature falls to 0 deg.  C. If the compressed air of a bicycle tire is allowed to expand and a sensitive thermometer is held in the path of the escaping air, the thermometer will show a decided drop in temperature.

The low temperature obtained by the expansion of air or other gases is utilized commercially on a large scale.  By means of powerful pistons air is compressed to one third or one fourth its original volume, is passed through a coil of pipe surrounded with cold water, and is then allowed to escape into large refrigerating vaults, which thereby have their temperatures noticeably lowered, and can be used for the permanent storage of meats, fruits, and other perishable material.  In summer, when the atmospheric temperature is high, the storage and preservation of foods is of vital importance to factories and cold storage houses, and but for the low temperature obtainable by the expansion of compressed gases, much of our food supply would be lost to use.

92.  Unexpected Transformations.  If the pressure on a gas is greatly increased, a sudden transformation sometimes occurs and the gas becomes a liquid.  Then, if the pressure is reduced, a second transformation occurs, and the liquid evaporates or returns to its original form as a gas.