larly explained. If the gas beneath the piston be heated the velocities with which the molecules are animated become increased. The energy with which they rap on the piston likewise becomes greater, in other words the effect of heating a given volume of gas is to increase its pressure. We thus see how the well-known properties of gases can be completely accounted for on the supposition that their constitution is precisely that which the molecular theory affirms.
As the gaseous molecules dart about they frequently come into collision. The effect of such a collision or encounter, as it is more properly called, is to deflect each of the molecules from the rectilinear path, which it had previously pursued, and to send it off in a new direction. These collisions take place with such frequency that in gas, at the ordinary temperature and pressure, each molecule experiences them at the rate of millions in a second. The path of each molecule thus consists of the free parts, during which it is practically uninfluenced by other molecules, and the disturbed parts during which it is acted upon by the molecules with which it has been fortuitously brought into collision. The frequency of these encounters depends, among other things, upon the density of the gas. In gas of the utmost rarity, such as that which is contained in the vessels employed by Mr. Crookes for his radiometers, the free path of the molecule may be as much as a quarter of an inch before it is turned aside in an encounter.
The actual parts of the molecule are themselves in a state of active vibration, and the nature of that vibration is often of a highly complex character. Notwithstanding the extreme minuteness of the vibrating particles, we are
