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 whirl 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 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.