we have generally the production of cold. The following table exhibits some of the best known freezing mixtures:

If the substances used and the apparatus have both been previously cooled down, still lower temperatures may be obtained.

96. Influence of pressure upon solution. Mr. Sorby (Proceedings of the Royal Society, vol. xii., April 30, 1863) has found that pressure exercises upon the solubility of salts an influence analogous to that which it exerts upon the melting points of bodies. Thus, when the united volume of the water and of a salt after solution is less than that of the water and salt separately before solution, or, in other words, where solution has diminished the volume, he finds that the effect of pressure is analogous to that which takes place where ordinary fusion diminishes the volume. In this case the solubility is increased by pressure, just as in the corresponding case the liability of ice to melt is increased by pressure (see Art. 92). Again, where solution has increased the volume (as, for instance, where sal-ammoniac is dissolved in water), pressure lessens instead of increasing the solubility.

PASSAGE FROM THE LIQUID TO THE SOLID STATE,
OR SOLIDIFICATION.

97. Substances which do not change their composition in passing from the liquid to the solid state.

We have here two laws of the same nature as those which regulate fusion.

1. Every substance under ordinary circumstances solidifies at a fixed temperature, which is the same as that of fusion.

2. The temperature of the liquid remains at this constant point from the time when solidification commences until it is complete.

If a liquid be allowed to cool very slowly, in becoming solid it often assumes the crystalline form, but most frequently we have the vitreous or amorphous state. The crystalline is, however, the most natural condition, and it will always be assumed when the particles have sufficient time to fall into their proper place; and even after substances have become solid molecular change in the direction of crystallization often takes place. Thus brass or silver if repeatedly heated and cooled becomes brittle, and exhibits a crystalline structure. In like manner, a cannon that has been often fired will at last burst in consequence of a change of this kind; and the vibrations to which the axles of railway carriages are liable gradually destroy the fibre and toughness of the iron, rendering it crystalline and brittle.

98. It is possible to lower the freezing point by various means. Thus pressure acts in lowering the freezing point of water just as it acted (Art. 92) in lowering the melting point of ice.

Again, water deprived of air and allowed to cool very slowly and without agitation may be reduced to -6°C while still retaining its fluid state, and if it be enclosed in a tube, its surface covered with a film of oil, and the pressure of the atmosphere withdrawn, it may be reduced to -12°C: but under these circumstances the smallest agitation or the presence of a crystal of ice produces solidification. Very frequently a glass vessel filled with water may be found in this state on a cold morning, when the addition of a bit of ice

in a very few seconds changes entirely the appearance of the liquid.

This sudden formation of ice is accompanied by a rise of temperature of the whole liquid, which mounts to the freezing point of water. The reason of this is, that ice requiring much less heat than water, leaves a quantity of heat free to raise the temperature of the whole liquid.

A very rapid agitation, or any other cause which, exerting an action upon the molecules, hinders them from assuming the requisite arrangement, retards the formation of ice. Capillary attraction acts in this way; and M. Despretz has found that in fine capillary tubes water may be lowered to -20°C without solidification. This circumstance probably explains why the sap is not oftener frozen in the capillary vessels of plants.

99. The great amount of the latent heat of water, combined with the fact that ice is lighter than water, are facts of great importance in the economy of nature. To make this clear let us see what occurs when a lake is frozen, supposing that the cold influence or abstraction of heat takes place over the surface of the lake. As the upper layer of water is cooled down it becomes heavier and sinks to the bottom, being replaced by a warmer and lighter layer from below: this process will go on until the whole water of the lake is reduced to 39° Fahr., the point of maximum density of water. When this temperature has been reached the process above described is at an end, and any further cooling of the upper strata will not cause them to sink, since they become specifically lighter below 39°.

When the surface of the lake has been cooled down to 32° Fahr. it will begin to freeze, but the process of freezing will go on very slowly, since a great quantity of heat must be taken from water before it becomes ice.

Again, when a layer of ice is once formed it does not sink

to the bottom, but remains on the top, so that the cooling influence can only freeze a second layer through the substance of the first, and so on. The ice formed thus protects the water below, which remains at 39°, a temperature which is not destructive to animal life.

100. Regelation. Faraday was the first to observe a very curious property of ice. Two pieces of thawing ice if put together adhere and become one; and this adhesion will take place in air or in water, or in vacuo. It would also seem to be independent of the application of pressure; and, provided the surfaces be smooth, when they are brought into the slightest contact, regelation ensues. Nor is it necessary that both surfaces be ice, for wool may be made to adhere to a block of thawing ice after the manner of regelation. The same thing takes place when a snowball is formed.

101. Probably the true explanation of this phenomenon is that advanced by Professor Forbes. He adopts the idea of the gradual liquefaction of ice which was deduced by Person from Regnault's experiments on latent heat, and supposes that true hard ice does not pass at once into water, but that there are intermediate stages in the process of liquefaction. The temperature of true hard ice is by this hypothesis essentially somewhat less than that of ice-cold water, and the substance corresponding to the intermediate temperature is supposed to appear in a slightly viscous or plastic state, being as yet neither quite solid nor quite liquid, and also to possess probably less than the latent heat of perfectly fluid water. In fact, ice in melting is here supposed to be similar to sealing-wax or wrought iron, both of which substances require a considerable range of temperature in order to pass from the solid to the liquid state, while we may imagine that the whole latent heat is not required until perfect fluidity is reached. The difference between ice and wrought iron in melting would therefore be one of abruptness of transition.

In ice the change is accomplished throughout a very small temperature range, in iron it requires a very large one.

The

subjoined figure willapproximately represent the state, as regards temperature, of a cubical block of thawing ice on this hypothesis. 102. Our conception of latent heat (Art. 91) will require to be somewhat modified in order to suit the hypothesis of gra

dual liquefaction, and we may represent to ourselves what takes place by means of the following diagram (Fig. 21).

B

"

B

A

B

C

D

Fig. 21.

"

Let the whole range of temperature between the commencement and the end of the process of liquefaction be denoted by AD, and subdivided into equal parts AB, BC, CD; also let AA' denote the whole heat of the body at temperature A; and supposing there were no such thing as latent heat, let BB' denote the heat of the body at temperature B, CC' its heat at temperature C, and DD' its heat at temperature D. The latent heat will however have to be added to these heats, in order to ex

press the total heat of the body at the various temperatures. Expressing this latent heat, which is supposed to increase gradually between the two temperatures A and D, by B'B", C'C", D'D", we have the whole lines BB", CC", DD"