that their mutual attractions are relatively unimportant. With lowering of temperature, the distance and rapidity of motion of the molecules diminish until under certain conditions, the attraction of the molecules for one another predominates, resulting in a much closer packing, and the appearance of the liquid form. The molecules, however, still retain a certain freedom of motion, but this is diminished with lowering of the temperature until at a certain stage the molecules form a tighter grouping, corresponding to the solid state where the freedom of motion of the individual molecules is much restricted. In order to account for the resistance of solids to compression or extension, it has been supposed that the force between molecules is attractive at large distances but repulsive at small distances. While we are able to offer a general explanation of the passage of an element from one state to another, a complete explanation of such phenomena will only be possible when we know the detailed structure of the atoms and the nature and magnitude of the forces between them.
While the kinetic theory of gases has proved very successful in explaining the fundamental properties of gases, its strength, and at the same time its weakness, lies in the fact that in most cases it is unnecessary for the explanation to know anything of the structure of the atom or molecule, or of the forces between them. In some investigations, in order to explain some of the more recondite properties of gases, assumptions have been made of definite laws of force between the molecules, but no very definite or certain results have so far been achieved in this direction. It should, however, be pointed out that the kinetic theory afforded us for the first time with a satisfactory method of estimating approximately the dimensions of molecules and the actual number in a given weight of matter. As the recent development of science has provided us with more certain methods of estimation of these important quantities, we shall not enter further into the question at present.
Crystals
There is another very striking form that matter sometimes assumes, which has always attracted much attention and which has recently emerged into much prominence. It is well known that the majority of substances under suitable conditions form crystals of definite geometrical form, which is characteristic of the particular atoms or groups of atoms. The great variety of crystal forms that are known have all been classified as belonging to one or more of the 230 forms of point symmetry which are theoretically possible. While considerations of symmetry are a sufficient guide to the classification of crystals, they offer no explanation of the definite architecture of the crystal nor of the nature of the forces that cause the atoms or molecules to arrange themselves in such definite geometric patterns. We are inevitably led
