ICE
, a brittle transparent body, formed of some fluid, frozen or fixed by cold. The specific gravity of Ice to water, is various, according to the nature and circumstances of the water, degree of cold, &c. Dr. Irving (Phipps's Voyage towards the North Pole) found the densest Ice he could meet with about a 14th part lighter than water. M. de Mairan found it, at different trials, 1-14th, 18th, or 19th lighter than water; and when the water was previously purged of air, only a 22d part.
The rarefaction of Ice has been supposed owing to the <*>ir-bubbles produced in Ice while freezing; these, being considerably large in proportion to the water frozen, render the Ice so much specifically lighter. It is well known that a considerable quantity of air is lodged in the interstices of water, though it has there little or no elastic property, on account of the disunion of its particles; but upon these particles coming closer together, and uniting as the water freezes, light, expansive, and elastic air-bubbles are thus generated, and increase in bulk as the cold grows stronger, and by their elastic force bursts to pieces any vessel in which the water is closely contained. But snow-water, or any water long boiled over the fire, affords an Ice more solid, and with fewer bubbles. Pure water long kept in vacuo and frozen afterwards there, freezes much sooner, on being exposed to the same degree of cold, than water unpurged of its air and set in the open atmosphere. And the Ice made of water thus divested of its air, is much harder, more solid and transparent, and heavier than common Ice.
But M. de Mairan, in a dissertation on Ice, attributes the increase of the bulk of the water under this form, chiefly to a different arrangement of its parts: the icy skin on water being composed of filaments which are found to be joined constantly and regularly at an angle of 60°, and which, by this disposition, occupy a greater volume than if they were parallel. Besides, after Ice is formed, he found it continue to expand by cold; a piece of Ice, which was at first only a 14th part specifically lighter than water, on being exposed some days to the frost, became a 12th part lighter; and thus he accounts for the bursting of Ice in ponds.
It appears from an experiment of Dr. Hooke, in 1663, that Ice refracts the light less than water; whence he infers, that the lightness of Ice, which causes it to swim in water, is not produced merely by the small bubbles which are visible in it, but that it arises from the uniform constitution or general texture of the whole mass: a fact which was afterward confirmed by M. de la Hire. See Hooke's Exper. by Derham, p. 26, Acad. Per. 1693, Mem. p. 25.
Sir Robert Barker thus describes the process of making Ice in the East Indies, in a country where he never saw any natural Ice. On a large plain they dig three or four pits, each about 30 feet square, and 2 feet deep; the bottoms of which are covered, about 8 or <*>2 inches thick, with sugar-cane, or the stems of the large Indian corn, dried. On this bed are placed in rows a number of small shallow unglazed earthen pans, formed of a very porous earth, a quarter of an inch thick, and about an inch and a quarter deep; which, at the dusk of the evening, they fill with soft water that has been boiled. In the morning before sunrise the Ice-makers attend at the pits, and collect what has been frozen in baskets, which they convey to the place of preservation. This is usually prepared in some high and dry situation, by sinking a pit 14 or 15 feet deep, which they line first with straw, and then with a coarse kind of blanketing. The Ice is deposited in this pit, and beaten down with rammers, till at length its own accumulated cold again freezes it, and it forms one solid mass. The mouth of the pit is well secured from the exterior air with straw and blankets, and a thatched roof is thrown over the whole. Philos. Trans. vol. 65, p. 252.
