The properties of the atom may thus be divided into two classes; in one class we have the properties, such as its weight and its Röntgen radiation, which are intrinsic to the atom, and which it carries with it unchanged into any compound of which it may be a constituent; in the other class we have the properties, such as the chemical properties of the atom, which depend upon its environment and upon the physical conditions, such as temperature, to which it is subjected. From the point of view of the structure of the atom, the properties of the second class depend upon the conditions of the surface of the atom; close to the surface there are small negatively electrified particles, which can be detached from the atom by agents at our disposal, and the properties of the atom modified thereby: the properties of the first class depend upon the structure of the innermost parts of the atom where there are also these negatively electrified particles, which are, however, so firmly held that they are not loosened by any chemical treatment it is in our power to apply to the atom.
For some time after Dalton's enunciation of his theory, no very important advances were made in our knowledge of atoms, but in the second half of the nineteenth century the Atomic Theory was greatly advanced by the work of Clausius, Clerk-Maxwell, Boltzmann, Joule, Kelvin, and Willard-Gibbs on the Kinetic Theory of Gases. These philosophers showed that many of the properties of gases can be explained on dynamical principles if the gas is regarded as a collection of a very large number of small particles in rapid motion. Though some important results as to the size of atoms were obtained in this way, the greater part of the work related to the properties of swarms of atoms, and threw but little light on the constitution of the individual atom. In fact, it was