Note:  Do not rely on this information. It is very old.


Gas is a substance whose particles exhibit et tendency to separate from one another. If a quantity of gas be introduced into a closed vessel, that quantity will immediately fill the vessel, retaining neither its original form nor its original volume. A gas may therefore be regarded as a substance possessing no rigidity; the smallest forces may deform it, and a removal of those forces will not necessarily be accompanied by a, return to the original shape. This special property is shared with liquids, from which gases are distinguishable by having a very great elasticity of bulk. Small forces may produce very great change in volume. The gaseous condition seems to be one in which the particles of the substance are endowed with it greater amount of kinetic energy than they possessed when the substance was in the liquid form. They are therefore able to separate themselves and to travel about independently in rectilinear courses, except in so far as collisions with other particles may alter their lines of motion. The particles of a given quantity of gas at a certain definite temperature possess a certain amount of momentum, and if this is altered in direction by their collision with the sides of the containing vessel the incessant impact produces a fairly constant and uniform pressure on the sides of the vessel. We say fairly constant, because the particles have not all got exactly the same speed, nor are they all exactly the same distance apart; but, taking them as a whole, they may each be regarded as possessing an average amount of energy, an average speed, and an average distance between each other. If the vessel be diminished in size, the number of impacts it receis es per second will be increased, and for the same temperature as before the gas exerts a greater pressure. This is the basis of the kinetic theory of gases developed by Clausius and Clerk-Maxwell which now receives the general support of physicists. Both theoretical and practical considerations have shown that the pressure is inversely proportional to the volume for a given quantity of gas at a constant temperature. If r is the volume of the gas and p its pressure per unit area, the product vp will remain a constant, through a wide range of pressures. This law, which is known in England as Boyle's Law (q.v.), is not perfectly true, considerable error being found when the pressures are great and the particles of gas too closely aggregated. "The second important law of gases is clue to Gay-Lussac, who showed that the increase of volume whicn a quantity of gas undergoes when its temperature is increased 1, in order to maintain constant pressure, is a fixed proportion of its initial volume at 0 C. This is expressed mathematically by the formula r, = r, (1 -+- at), where r is the volume at the temperature t degrees, v0 that at 0 C, and a a constant that is practically the same for all gases, being about ji-j. This constant is called the coefficient of expansion.