onium, which, if simply dissolved
in rather less than its own weight of water, reduces the temperature
about twenty-five degrees below freezing. The objections to its use
are that its frigorific power is not sufficiently great to freeze
readily; and if it be required to form dessert ices, it is requisite
to renew the process, at the expiration of a quarter of an hour, a
second time, and, if the weather is very hot, and the water used is
rather warm, even a third or fourth time. Again, nitrate of ammonium
is a very expensive salt; even in France, where it is manufactured
expressly for this purpose, it is sold at the rate of three francs a
pound; and in England it cannot be obtained under a much higher price.
One great recommendation, however, attends its use, namely, that it
may be recovered again, and used any number of times, by simply
boiling away the water in which it is dissolved, by a gentle fire,
until a small portion, on being removed, crystallizes on cooling.

2152. Washing Soda as a Freezing Mixture.

If, however, nitrate of ammonium in coarse powder is put into the
cooler, and there is then added twice its weight of freshly crushed
washing soda, and an equal quantity of the coldest water that can be
obtained, an intensely powerful frigorific mixture is the result, the
cold often falling to forty degrees below freezing. This is by far
the most efficacious freezing mixture that can be made without the use
of ice or acids. But, unfortunately, it has an almost insuperable
objection, that the nitrate of ammonium is decomposed by the soda, and
cannot be recovered by evaporation; this raises the expense to so
great a height, that the plan is practically useless.

[ALL IS NOT GOLD THAT GLITTERS.]

2153. Sal Ammoniac as a Freezing Mixture.

If the ordinary sal ammoniac of the shops is used, it will be found
both difficult to powder, and expensive; in fact, it is so exceedingly
tough, that the only way in which it can be easily divided, except in
a drug mill, is by putting as large a quantity of the salt into water
which is actually boiling as the latter will dissolve; as the solution
cools, the salt crystalizes out in the solid form, and if stirred as
it cools, it separates in a state of fine division. As this process is
troublesome, and as the sal ammoniac is expensive, it is better to use
the crude muri