GASES, Kinetic Theory of. The theory which regards gases as aggregates of discrete particles (or °molecules))) of matter that are incessantly flying about and colliding with one another, the space in which they are moving being presumably absolutely vacuous, save for the omnipresent luminiferous ether. (See ETHER). According to this theory, the mole cules which are in the outer parts of a given mass of gas must beat incessantly upon the walls of the containing vessel, flying back again from these walls in the same way that they fly away from one another after collisions among themselves. This being the case, it is plain that the walls of the containing vessel are in the same condition as a target against which a furious storm of bullets is striking perpetually. Such a storm of bullets would tend to force the target in the direction in which the bullets were moving before collision; and if the impacts were frequent enough, they would have an effect upon the target which could not be distinguished from a continuous pressure. And if we pass, in thought, from target to retaining vessel, and from bullets to molecules, we shall have a good conception of the kinetic theory of gaseous pres sure. Before the behavior of molecular aggre gates can be studied by mathematical methods, it is•necessary to make certain assumptions with regard to the nature of the molecules. Some of the received assumptions have been made on account of their apparent necessity, and others have been made for no reason whatever, except that they simplify the mathematical treatment of the problems that arise. Thus molecules are assumed to be perfectly elastic, because it has been held to be evident that if they were not so, their incessant collisions must result in a grad ual loss of velocity, which would not cease un til they were all at rest. The assumption of per fect elasticity is therefore commonly regarded as a logical necessity, since we do not observe any tendency toward rest among the molecules of gases; that is, we do not perceive any tend ency toward a fall of pressure, in a gas that is isolated, thermally and otherwise, from its environment. In the earlier mathematical in
vestigations of the properties of gases, from the standpoint of the kinetic theory, the mole cules were assumed, furthermore, to be ex ceedingly small (practically mere physical points), and they were considered to be hard, smooth and spherical and to exert. no influence upon one another when not in actual contact; these assumptions being made, not because it was considered to be in the least degree likely that molecules have such properties, but merely in order to lessen the mathematical difficulties involved in the subsequent analysis — difficul ties that are serious enough, even when the problem is made as simple as possible. For example, they were assumed to be hard, in order that collisions might be considered as having no sensible duration. They were as sumed to be exceedingly small, in proportion to the space in which they move, in order that the probability of a collision in which three or more molecules should come together at once might become vanishingly small in comparison with the probability of a collision in which the molecules come together in pairs, the discus sion of the more complex collisions being thereby avoided. They were assumed to be spherical, because spheres can collide with each other in only one way; whereas other bodies (cubes, for example) can come together in the greatest variety of ways, according to their relative orientation at the moment of collision. They were assumed to be smooth, in order to avoid the necessity of taking account of the rotations that are produced when rough spheres glance against one another obliquely. The as sumptions stated above were adopted in what may be called the Maxwellian period of the development of the kinetic theory and Max well and other mathematicians made elaborate investigations of the behavior of a practically in finite number of molecules having these prop erties, when once set in motion in a finite space.