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[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. 2154. Coldness of the Materials used. It is hardly necessary to add, that in icing wines, or freezing, the effect is great in proportion to the coldness of the materials used; therefore, every article employed, viz., the water, tubs, mixtures, &c., should be as cool as possible. 2155. Blackbirds. The cock bird is of a deep black, with a yellow bill. The female is dark brown. It is difficult to distinguish male from female birds when young; but the darkest generally are males. Their food consists of German paste, bread, meat, and bits of apple. The same treatment as given for the thrush (See par. 2456) applies to the blackbird.