SOLIDIFICATION, the passage of a sub stance into the solid state, either from a state of solution or fusion, or directly from the gase ous state. The term is usually applied, however, to the transition from the melted state into the solid form, and it is only in this sense that it will be here discussed. Some substances which are stable in the solid form cannot be melted without undergoing decomposition, either par tial or complete; but many others can be con verted into the liquid form by simple heating, passing back into the solid form again when the heat is abstracted. In general, fusion im plies an addition of heat energy, and solidifi cation implies its abstraction. Thus, to freeze one pound of water it is necessary to abstract from it an amount of heat energy that would be sufficient to raise the temperature of a pound of liquid water by about 140 degrees of the Fahren heit scale; and in order to melt the pound of ice a precisely equal quantity of heat energy must be added to it again. All liquids may be divided into two general classes, whose laws of solidification are apparently very different. The first class includes bodies such as wax and glass, which, when they are cooled, pass into the solid state by a transition so gradual that it is impossible to assign any definite temperature at which the transformation can be considered to take place. The other class includes substances which, like water, do not begin to solidify until they have been cooled to a certain definite tem perature peculiar to each substance. Solidifica tion then begins at once, and it progresses continually, as the heat is abstracted, the tem perature of the mass remaining unchanged until all of the liquid has been transformed into the solid form. In some cases the solid body that is obtained is crystalline in structure, and in other cases it is amorphous. In general, the temperature or solidification of a substance is identical with the melting point of the same substance; but in some cases the molten condi tion persists until the mass has been cooled to a temperature materially lower than the tempera ture of fusion. The rare metal known as gal
lium, for example, melts at 86° F.; but when once melted it may be cooled to about 36° F. before it solidifies. This phenomenon — the persistence of the molten condition after the substance has been cooled below the normal melting point —is called surfusion, and it may be regarded as analogous to the supersaturation that is sometimes observed in solutions (q.v.). If a small fragment of solid gallium be added to the molten metal at a temperature below 86° F., solidification at once ensues; and in general it may be said that in any fluid substance in a state of surfusion solidification is at once in duced by the addition of a fragment of the same substance in a solid form. Some sub stances increase in volume when they pass from the molten to the solid state and others de crease. For example, when water solidifies it increases in volume so that the bulk of the ice that is formed is about one-eleventh greater than that of the original water. Bismuth, bronze and a few other metals which give good sharp castings also maintain an increase in vol ume upon solidification. Lead, iron and most metals, on the other hand, decrease in volume when fusing from the liquid to the solid state.
Pressure has a marked influence upon the temperature of solidification; and Dr. James Thomson showed, from theoretical principles, that under great pressures the temperature of solidification is lowered in bodies which ex pand upon solidification, and raised in those in which solidification is accompanied by contrac tion; this theoretical prediction having been subsequently verified by direct experiment.