SOLID STATE, THEORY OF. It is often said that a solid has volume and shape, a liquid volume, but no shape, and a gas neither definite. volume nor shape. Though it sums up neatly the main characteristics of the three states of aggregation, this statement scarcely forms an accurate or adequate definition. The volume of both solids and liquids can be modified, if only to a slight degree, by external pressure. Similarly the shape of a solid can be changed by applied forces. Conversely, a liquid unstrained, has the tendency to assume a spherical shape, though it is true that this tendency is slight. Again large masses of gas under the influence of the mutual attraction of the molecules, tend to form those vast spheres which we know as stars. The definitions, such as they are, describe in reality the viscosity and surface tension, rather than the state of aggregation. Never theless, they form a rough working picture upon which a more searching examination may be based.
From the molecular point of view the three states of aggre gation of matter may be described in terms of the forces acting between the molecules. Besides the ordinary gravitational attrac tion, there is as is well known, a molecular force of attraction or cohesion which comes into play at distances small compared to our ordinary standards, but large compared to the molecules themselves. This force of attraction which is probably electro magnetic in origin, may be represented by the curve A as shown in fig. I, which diminishes with considerable rapidity as the distance between the molecules measured along the abscissa increases. When the molecules approach very close to one another there is a force of repulsion which philosophers, though not per haps physicists, might attribute to the unscientific axiom, that two things cannot occupy the same place at the same time. This force is undoubtedly closely related to what may be called the shape of the molecule and is presumably due to the mutual repul sion of the peripheral negative electric charges. Whatever its origin, it is undoubtedly very small until the molecules approach one another within a distance comparable with their own diameter where it rapidly becomes extremely large. If this force be repre sented by curve B then it is evident that the interplay of the attraction and repulsion lead to a force of repulsion when the molecules approach one another within a given distance which changes over to a force of attraction as this distance is exceeded.
On increasing the distance the attraction increases until it attains a maximum value after which it rapidly diminishes to become negligible at distances large compared to the molecular diameter.
The existence of forces between molecules which may be rep resented by curves of this type explains the existence of the three states of aggregation to matter. If the energy of the molecules is large compared to the total energy necessary to separate them from their closest approach to infinity, then the substances will be in the gaseous state for the collisions will be so violent that the particles rebound without coalescing. When the energy is diminished the violence of the collisions decreases and occasions will arise when the molecules coalesce and ultimately condense to a liquid. In the liquid the molecules move about rapidly colliding with one another, but only in exceptional cases have they enough energy to overcome the attraction of the sur face molecules and to escape into the vapour phase. Their velocities being distributed, according to Maxwell's expression, there will always be a few with the necessary abnormally large energy; in general however these form only a small fraction of the whole, a fraction which varies of course with the temperature and is a measure of the vapour pressure.
It may be remarked that the energy required by a molecule, to enable it to overcome the attraction of all its neighbouring mole cules at the surface of a liquid is greater (though only about three times greater) than the work necessary to separate two molecules from contact to infinity. This explains the phenomenon of supersaturation. If no drop of liquid is present, only molecules will coalesce whose relative speed when they collide is so small as to be insufficient to overcome their mutual attraction. As soon as a drop of liquid is formed all these comparatively slow molecules will condense upon it ; for to escape from the attrac tion of all the molecules in a surface requires considerably more energy than to prevail over the attraction of a single molecule.