2151. Nitrate of Ammonium as a Freezing Mixture
Another substance, which is free from any corrosive action or unpleasant odour, is nitrate of ammonium, 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 muriate of ammonium, which is the same substance as sal ammoniac, but before it has been purified by sublimation. This is not usually kept by druggists, but may be readily obtained of any of the artificial manure merchants, at a very moderate rate; and its purity may be readily tested by placing a portion of it on a red-hot iron, when it should fly off in a vapour, leaving scarcely any residue.