For a remarkable case of slow thawing, and illustration of tho prin ciple on which the gradual nature of that process depends, we may refer to some experiments made by Dr. Faraday for the investigation of a different subject, that of regelation. He prepared a bath of water, which could be retained, by appropriate contrivances, at the unchanging temperature of Fahr., or the melting-point of ice, for a week or more ; but a small piece of ice floating in it for that timo was not entirely melted away. Yet the temperature was adequate to the liquefaction of ice, for a very slow thawing process was really going on in the bath during the whole time, na was rendered evident by the state of this very piece of ice ; but the glass jar containing the water being surrounded by a system of bad conductors of heat, including dry flannel and broken ice, the heat of the atmosphere and exterior sur rounding bodies could only very slowly penetrate to the ice in the bath, and there supply the heat required to become latent in the process of thawing it, and hence the rate of thawing was so slow as not to dissolve a cubic inch of ice in six or seven days. (See Proc. of Royal Society' for April 26, 1860, vol. x., pp. 442, 449.) A phenomenon observed at Inspruck, in the Tyrol, which has been referred to the contrary operation of local winds, probably admits of explanation upon the principle of the absorption or rendering latent of heat in the thawing process. At that place the snow is often seen melting on the mountains above the town, at an elevation of 3000 feet, while it continues very cold and there is not the least sign of thaw in the valley of the river Inn below. It is then popularly said that the south wind is driving the cold into the valley ; and this toying seems to involve the truth. The air above will be greatly cooled down by the abstraction of its heat in the melting of the snow, and, thus becoming heavier, will descend in the atmosphere and maintain for a time the low temperature in the valley below.
The overflowing of riven! by the dissolution of the snow and ice on the mountains above their sources is well known [litvens, col. 119J, and to the liquefaction of the ice formed by the previous congelatien of water which has Introduced itself into the fissures of rocks is to be ascribed the occasional severance of large masses from the sides of mountains; the expansion of the water in freezing having destroyed the cohesion, so that the Farts are only held together by the ice, and on the liquefaction of this the disunion is complete.
Two pieces of thawing ice, if brought into contact, adhere and become one ; at a place where liquefaction was proceeding, congelation suddenly occurs. This is the phenomenon and process of regelan'ett, already treated of in the article lea. The view of the thawing or
melting of ice taken by M. Person, that it is a gradual process, resembling that of wax and metals, and not really a sudden one, and its adoption by Professor James Forbes, as well as the objections to it urged by Professor James Thomson, have been noticed in the article Ice. Person's original paper, On the Latent Heat of Fusion of Ice,' will be found in the Comptes Rendus' of the Royal Academy of Sciences of Paris, fur April 29, 1850, vol. xxx., p. 526. The evidence he adduces of the supposed viscosity of ice, intermediate between the states of rigid solidity and perfect fluidity, consists solely of an amount of latent heat which a high authority agrees with us in considering to be much within the probable errors of the delicate experiments required. The application made by Professor Tyndall of the principle of regelation, renders the plasticity of ice in the mass quite intelligible, without the necessity of attributing a viscous property to that substance, the existence of which is negatived by all the other properties which it pee...caeca.
Although the terms thawing and freezing were originally applied only to the solidification and re-liquefaction of water, by variations of temperature, and derivatively to those of other fluids in ordinary use, their sense has become extended in the progress of that more precise knowledge of natural things which is called science, to denote also, generally, the solidification and re-liquefaction of liquids not commonly observed in the solid condition. We speak, for example, of the freezing and thawing of mercury ; and not only so, but the terms thawing and freezing are sometimes used to express the circumstances of the melting and re-solidification of bodies which are ordinarily solid— solid, that is, at common temperatures, an expression whicb, as usually employed, includes a considerable thermometric range from above the freezing to below the boiling point of water, and is even extended, though indefinitely, somewhat below the former and considerably above the latter, though not to a red heat, or a temperature at which light is evolved. Thus these terms have been applied to the melting and crystallisation of glass. In like manner, the term fusion, originally employed with reference to the liquefaction of bodies which are solid at common or much higher temperatures, has come to be applied, as equivalent to that of thawing, to the melting after solidification of substances which aro ordinarily liquid. In this manner we speak of the fusion of ice, as well as that of wax or of copper; and Dr. Faraday has described, in the paper referred to above, an experi ment in which layers of ice are produced "of greater and less