A neutral salt.
Saltpetre is a neutral salt, the constituents of which are 46.55 potash, and 53.45 nitric acid; the latter consisting of two volumes nitrogen and five of oxygen. It is white, and of a fresh, sharp, and slightly bitter taste. It crystallizes in six-sided prisms. Exposed to the air, it remains permanent unless impure, or that the atmosphere is very moist.
NEW METHOD.
New method.
Forty cwt. of the grough saltpetre is put into a copper with 270 gallons of water, and treated in precisely the same way as we have before described for the first refining; it is then filtered and run off into large troughs, about 10 feet long by 6 feet wide, and 9 inches deep, lined with sheet copper; this liquor is then kept in a state of agitation by a wooden rake, until nearly cold. By this process a large quantity of very minute crystals are formed, which are collected as they form by a wooden hoe, and shovelled with a spade on to a framework covered with copper sieving resting on the opposite sides of the trough, and allowed to drain. These fine white crystals, which have exactly the appearance of snow, when they have drained sufficiently, are raked over in a washing cistern adjoining, which is about 6 feet long, 4 feet wide, and 3ft. 6in. deep, and fitted with a false wooden bottom that can be removed at pleasure. Cold water is allowed to run on to the saltpetre in this cistern till it is nearly level with the top. After remaining for an hour it is drained off, and filled again with fresh water, which is drained off after about another hour. The saltpetre thus obtained is perfectly pure, and equal in every respect to the treble-refined by the old method. The water remaining in the cisterns after agitation, is left till the next morning, when a quantity of larger crystals are formed on the bottom and sides; these are equal to once-refined by the old method, and are used with grough; the mother-liquor is then drained off, and evaporated in the usual way. The water from each washing is conveyed into cisterns, and used with grough saltpetre instead of water; but, as it contains a small portion of saltpetre in solution, a lesser quantity of grough is used to make the proportions correct.
Drying.
The saltpetre flour, however, contains a certain degree of moisture, which has to be dried off in the following way: two large copper trays, about 10 feet by 6 feet, with a 3-inch rim, are fixed over flues heated by a furnace, 4 inches of sand being between the flues and the bottom of the trays; the saltpetre is spread about 2 inches deep all over, and raked about till dry; it is then barrelled up for use. It takes about two hours to dry 5 cwt.
Comparison of the two methods.
On comparing the two systems, there cannot for one moment be a doubt as to the immense advantages of the latter over the former. As an example, in the refinery where this new process is carried on, the result (that is to say, pure saltpetre) is obtained in one day instead of six, with less than one half the amount of labour and coals.