Gases and liquids have several properties in common, and some in which they seem to differ are in reality only different degrees of the same property. Thus, in both, the particles are capable of moving; in gases with almost perfect freedom; in liquids not quite so freely, owing to a greater degree of viscosity. Both are compressible, though in very different degrees.
If a liquid and a gas both exist under the pressure of one atmosphere, and then the pressure be doubled, the water is compressed by about the 1/20000 part while the gas is compressed by one-half. In density there is a great difference; water, which is the type of liquids, is 770 times as heavy as air, the type of gaseous bodies, while under the pressure of one atmosphere. A spiral spring only shows elasticity when it is compressed; it loses its tension when it has returned to its primitive condition. A gas has no original volume; it is always elastic, or in other words, it is always striving to attain a greater volume; this tendency to indefinite expansion is the chief property by which gases are distinguished from liquids.
Matter assumes the solid, liquid, or gaseous form according to the relative strength of the cohesive and repulsive forces exerted between their molecules. In liquids these forces balance; in gases repulsion preponderates.
By the aid of pressure and of low temperatures, the force of cohesion may be so far increased in many gases that they are readily converted into liquids, and we know now that with sufficient pressure and cold they may all be liquified. On the other hand, heat, which increases the _vis viva_ of the molecules, converts liquids, such as water, alcohol and ether or gas into the aëriform state in which they obey all the laws of gases. The aëriform state of liquids is known by the name of _vapor_, while gases are bodies which, under ordinary temperature and pressure, remain in the aëriform state.
In describing exclusively the properties of gases, we shall, for obvious reasons, refer to atmospheric air as their type.
_Expansibility of Gases._ This property of gases, their tendency to assume continually a greater volume, is exhibited by means of the following experiment:—A bladder, closed by a stop-cock and about half full of air, is placed under the receiver of the air pump, Fig. 337, and a vacuum is produced, on which the bladder immediately distends.