432. Another beautiful experiment bearing upon this point has recently been made by M. Boussingault. He filled a hollow steel cylinder with water and chilled it. In passing to ice, water, as you know, expands ([§ 45]); in fact, room for expansion is a necessary condition of solidification. But in the present case the strong steel resisted the expansion, the water in consequence remaining liquid at a temperature of more than 30° Fahr. below the ordinary freezing point. A bullet within the cylinder rattled about at this temperature, showing that the water was still liquid. On opening the tap the liquid, relieved of the pressure, was instantly converted into ice.
433. It is only substances which expand on solidifying that behave in this manner. The metal bismuth, as we know, is an example similar to water; while lead, wax, or sulphur, all of which contract on solidifying, have their point of fusion heightened by pressure.
434. And now you are prepared to understand Professor James Thomson's theory of regelation. When two pieces of ice are pressed together liquefaction, he contends, results. The water spreads out around the points of pressure, and when released re-freezes, thus forming a kind of cement between the pieces of ice.
[§ 63.] Faraday's View of Regelation.
435. Faraday's view of regelation is not so easily expressed, still I will try to give you some notion of it, dealing in the first place with admitted facts. Water, even in open vessels, may be lowered many degrees below its freezing temperature, and still remain liquid; it may also be raised to a temperature far higher than its boiling point, and still resist boiling. This is due to the mutual cohesion of the water particles, which resists the change of the liquid either into the solid or the vaporous condition.
436. But if into the over-chilled water you throw a particle of ice, the cohesion is ruptured, and congelation immediately sets in. And if into the superheated water you introduce a bubble of air or of steam, cohesion is likewise ruptured, and ebullition immediately commences.
437. Faraday concluded that in the interior of any body, whether solid or liquid, where every particle is grasped so to speak by the surrounding particles, and grasps them in turn, the bond of cohesion is so strong as to require a higher temperature to change the state of aggregation than is necessary at the surface. At the surface of a piece of ice, for example, the molecules are free on one side from the control of other molecules; and they therefore yield to heat more readily than in the interior. The bubble of air or steam in overheated water also frees the molecules on one side; hence the ebullition consequent upon its introduction. Practically speaking, then, the point of liquefaction of the interior ice is higher than that of the superficial ice. Faraday also refers to the special solidifying power which bodies exert upon their own molecules. Camphor in a glass bottle fills the bottle with an atmosphere of camphor. In such an atmosphere large crystals of the substance may grow by the incessant deposition of camphor molecules upon camphor, at a temperature too high to permit of the slightest deposit upon the adjacent glass. A similar remark applies to sulphur, phosphorus, and the metals in a state of fusion. They are deposited upon solid portions of their own substance at temperatures not low enough to cause them to solidify against other substances.
438. Water furnishes an eminent example of this special solidifying power. It may be cooled ten degrees and more below its freezing point without freezing. But this is not possible if the smallest fragment of ice be floating in the water. It then freezes accurately at 32° Fahr., depositing itself, however, not upon the sides of the containing vessel, but upon the ice. Faraday observed in a freezing apparatus thin crystals of ice growing in ice-cold water to a length of six, eight, or ten inches, at a temperature incompetent to produce their deposition upon the sides of the containing vessel.
439. And now we are prepared for Faraday's view of regelation. When the surfaces of two pieces of ice, covered with a film of the water of liquefaction, are brought together, the covering film is transferred from the surface to the centre of the ice, where the point of liquefaction, as before shown, is higher than at the surface. The special solidifying power of ice upon water is now brought into play on both sides of the film. Under these circumstances, Faraday held that the film would congeal, and freeze the two surfaces together.