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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 purpose 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. Many of the freezing mixtures which are to be found described in books are incorrectly so named, for although they themselves are below the freezing point, 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 ices, and cooling wines, &c. It consists of coarsely powdered Glauber salt (sulphate of sodium), on which is poured about two-thirds its weight of spirit of salts (hydrochloric acid).