REGELA'TION. This is an exceedingly ill-chosen term for a somewhat obscure pha nomenon, inasmuch as it implies a previous state which may not have existed. Unfor tunately, the term has come into general use, and we must make the best of it. The principal fact to be explained is the adhesion of two pieces of ice brought into contact, not merely in air, but even when both are immersed in water at such as 100° Fehr. Several explanations have been proposed, of which we may specially men tion those of Faraday, Forbes, and J. Thomson.
Faraday's idea seemed to be, that in liquid and solid bodies the proximity of particles in a particular state tends to produce the same state in other particles; and thus, that a film of water between two plates of ice tends to assume the solid state, There are many singular phenomena in physical science which arc apparently explicable by this sugges tion; but with all due deference to so great an-authority, the so-called explanation seems merely to Waft the difficulty, without in any way overcoming it.
Forbes starts with the assumption, that ice is essentially colder than water, and there fore that there is constantly a transfer of heat from water to ice which, is in contact with it; the effect being to cover the surface of the ice with a film of half-melted ice or half frozen water. Such a film, existing between two slabs of ice, would part will' heat to both, and would freeze without melting the adjacent ice. This explanation would be satisfactory if the postulate could be granted, but it seems very improbable that there is any such essential difference of temperature between solid and liquid water.
The explanation proposed by prof. J. Thomson is undoubtedly founded on a rem-a cause, but there may be some hesitation in allowing that the cause is adequate to the i production of the observed effect in every case. It is certain, however, that it accounts for at least part of the phenomenon. It is founded on his very beautiful theoretical dis covery that the freezing-point of water is lowered' by pressure, which was experimentally verified by W. Thomson. Hence, if two slabs of ice be pressed together. at the points
of greatest pressure the ice will be melted; its latent heat of fusion must be drawn from surrounding bodies, and thus cold is produced which will freeze part of the film of water between the two slabs. The points of greatest pressure will thus be shifted, and the process of inciting and regelation may go on indefinitely. Objections to this explanation were advanced by Faraday and Forbes, who showed that slabs of ice freeze together when suspended vertically with the view of avoiding pressure between them. But J. Thomson shows that the capillary forces of the film of water which must (in these cases) be between the slabs (for without directly applied pressure the effect cannot be obtained with slabs of dry ice), arc sufficient to produce the pressure requisite for the application of his mode of explanation.
This part of the subject cannot be said to be completely cleared up; but the theory of J. Thomson has been applied with perfect success to the explanation of the very extraordinary phenomena observed in glaciers (q.v.), where enormous forces are con stantly at work. It evidently at once accounts for the result of observation, due to Rendu and Forbes, that a glacier moves like a viscous fluid: in fact, it shows why and how the mass gives way to pressure, and how it is refrozen in a new form, which in turn gives way to the new distribution of pressure. The explanation of the veined structure, the formation of clear ice from snow, etc., are all easily deduced from it.
The phenomena of regelation are easily seen in the making of snow-balls, which is well known to be impossible, by the hands at least, when the snow has been exposed to great cold, and is therefore dry. But, even in this case, the effect is easily obtained by the application of pressure sufficient to melt the ice, as is well seen in wheel-tracks, etc. By means of a Bramah's press, it is easy to convert a snow-ball into a sphere of perfectly clear ice.