THAW is the reduction of ice or snow to a liquid state in conse quence of an increase in temperature. In the ordinary succession of the seasons, this effect is produced on the surface of the earth or in the atmosphere during the spring by the return of the sun to tho hemisphere of the observer, the solar rays then falling in greater abundance than before on a given extent of ground ; or it is produced by accidental currents of warm air which pass over a frozen mass. The dissolution of particles of ice or snow floating in the atmosphere, and the universal liberation of moisture previously frozen up, are the causes of the humidity which accompanies a thaw.
Ae the conversion of a liquid into ice always commences at the surface of the former, and about the sides of the vessel containing it, or about those of a solid body immersed in it, so in ice surrounded by air which has acquired a higher temperature than the ice has, the pro cess of liquefaction commences at the sides and extends gradually from thence inwards ; ice being a bad conductor of heat, the central parts of it, under ordinary circumstances, are the last which are dissolved. It is observed that when solid bodies, whose temperatures are equal to one another and higher than that of ice, are applied to the latter, the ice is dissolved most rapidly by those which have tho greatest power' of conducting caloric : thus a piece of ice being laid on a plate of polished metal, and a piece of equal magnitude, on wood, the ice on the metal will be dissolved before that which is laid on the wood, not only when the temperature of the metal and wood are equal, but even when the temperature of the wood considerably exceeds that of the metal, the latter conveying morn abundantly to the ice the caloric which it is continually receiving from the atmosphere.
A severe and long-continued frost abstracts so much caloric from terrestrial bodies, as the walls of buildings which are not exposed to the sun, that these are often cooled below the temperature of freezing water ; and while in this state, if a current of warm air pass over their surfaces, the aqueous vapour which the air contains deposits itself on the walls, where it is converted into ice or snow : it remains thus frozen for a time after a thaw has commenced, but at length, the temperature increasing, the ice is melted and the walls are then covered with moisture.
It is often remarked that, at the time of a thaw taking place, there is felt a degree of coldness greater than that which is experienced during the continuance of the frost; this has sometimes been ascribed merely to the evaporation of the moisture which is then on the skin, as the thermometer at the same time indicates an elevation of tempe rature in the atmosphere. But the evaporative power of the skin must depend both upon the temperature of the air and the amount of aqueous vapour which it already contains, and in part also on the local heat of the human body. The sensation alluded to arises, in all probability, from the continual conversion of the sensible heat of the atmosphere and the surface of the earth and bodies resting upon it, into a latent form during the process of thawing, the latent heat of water being greater than that of any other substance. Dr. Faraday has shown that the conversion of a cubic yard of ice simply into water at 32', or ice-cold, would absorb or render latent the whole heat emitted during the combustion of a bushel of coal. [HAIL; ICE; WATEU.j The amount of heat thus required in the liquefaction of ice renders a thaw so gradual. This is an important provision of nature ; but for it, "the ice that had accumulated during A long winter would at the first breeze from the south be instantly converted into water, and sweep before it, not merely the habitations of man and their tenants, but trees, rocks, and hills ;" every thaw, in fact, would occasion a frightful inundation. Conversely, frost is rendered gradual by the evolution of the latent heat of water in a sensible form.