The use of ice in cooling depends upon the fact of its requiring a vast quantity of heat to convert it from a solid into a liquid state, or, in other words, to melt it, and the heat so required it obtains from those objects with which it may be in contact. A pound of ice requires nearly as much heat to melt it as would be sufficient to make a pound of cold water boiling hot; hence its cooling power is extremely great. But ice does not begin to melt until the temperature is above the freezing point, and therefore it cannot be employed in freezing liquids, etc, but only in cooling them.

If, however, any substance is mixed with ice which is capable of causing it to melt more rapidly, and at a lower temperature, a still more intense cooling effect is the result; such a substance is common salt, and the degree of cold produced by the mixture of one part of salt with two parts of snow or pounded ice, is greater than thirty degrees below freezing. In making ice creams and dessert ices, the following articles are required: - Pewter ice-pots with tightly-fitting lids, furnished with handles;wooden ice-pails, to hold the rough ice and salt, which should be stoutly made, about the same depth as the ice-pots, and nine or ten inches more in diameter; each should have a hold in the side, fitted with a good cork, in order that the water from the melted ice may be drawn off as required. In addition, a broad spatula, about four inches long, rounded at the end, and. furnished with a long wooden handle, is necessary to scrape the frozen cream from the sides of the ice-pot, and for mixing the whole smoothly together.

When making ices, place the mixture of cream and fruit to be frozen, in the ice-pot, cover it with the lid, and put the pot in the ice-pail, which proceed to fill up with coarsely-pounded ice and salt, in the proportion of about one part of salt to three of ice;let the whole remain a few minutes (if covered by a blanket, so much the better), then whirl the pot briskly by the handle for a few minutes, take off the lid, and with the spatula scrape the iced cream from the sides, mixing the whole smoothly; put on the lid and whir again, repeating all the operations every few minutes until the whole of the cream is well frozen. Great care and considerable labour are required in stirring, so that the whole cream may be smoothly frozen, and not in hard lumps. When finished, if it is required to be kept any time, the melted ice and salt should be allowed to escape by removing the cork, and the pail filled up with fresh materials.

It is scarcely necessary to add, that if any of the melted ice and salt is allowed to mix with the cream the latter is spoiled. From the difficulty of obtaining ice in places distant from large towns, and in hot countries, and from the impracticability of keeping it any length of time, or, in fact, of keeping small quantities more than a few hours, its use is much limited, and many have been the attempts to obtain an efficient substitute. For this pur pose various salts have been employed which, when dissolved in water or in acids,absorb a sufficient amount of heat to freeze substances with which they may be placed in. contact. We shall not attempt in this article to describe all the various freezing mixtures that have been devised, but speak only of those which have been found practically useful, state the circumstances which have prevented any of them coming into common use, and conclude by giving the composition of the New feezing Preparation, which is now exported so largely to India, and the composition of which has hitherto never been made public.

Many of the freezing mixtures which are to be found described in books, are incorrectly so named, for although they themselves become colder than freezing, yet they are not sufficiently powerful to freeze any quantity of water, or other substances, when placed in a vessel within them. In order to be efficient as a freezing mixture, as distinguished from a cooling one, the materials used ought to be capable of producing by themselves an amount of cold more than thirty degrees below the freezing point of water, and this the ordinary mixtures will not do. Much more efficient and really freezing mixtures may be made by using acids to dissolve the salts. The cheapest, and perhaps the best, of these for ordinary use, is one which is frequently employed in France, both for making dessert ice's, and cooling wines, etc.

It consists of coarsely-powdered Glauber salt (sulphate of soda), on which is poured about two thirds its weight of spirits of salts (muriatic acid). The mixture should be made in a wooden vessel, as that is preferable to one made of metal, which conducts the external heat to the materials with great rapidity; and when the substance to be cooled is placed in the mixture, the whole should be covered with a blanket, a piece of old woollen carpet doubled, or some other non-conducting material, to prevent the access of the external warmth;the vessel used for icing wines should not be too large, that there may be no waste of the freezing mixture. This combination produces a degree of cold thirty degrees below freezing; and if the materials are bought at any of the wholesale druggists or drysalters, it is exceedingly economical. It is open, however, to the very great objection, that the muriatic acid is an exceedingly corrosive liquid, and of a pungent, disagreeable odour; this almost precludes its use for any purposes except that of icing wines.