When a sufficient quantity of the strong solution is obtained, it is put into the copper, or boiler, and evaporated. During the evaporation, there is a scum formed, and sundry earthy substances, in the form of a mud, are deposited. This is usually caught in a vessel placed in the boiler, which is raised from time to time, by means of a rope, moved through a pulley, and fastened to a chain from the handles of the vessel. The solution is concentrated until it indicates the strength of twenty-five degrees of Beaumé's areometer. It is then mixed with the mother water of the preceding boiling, and a concentrated solution of the potash of commerce is added, until the precipitation ceases. The sulphate of potassa may be used for the same purpose, at least to decompose the nitrate of lime; but it must be used in the first instance, and the operation finished in the common way, by the addition of potash. The precipitation being finished, that is to say, the nitrates of lime and magnesia, being transformed into nitrate of potassa, the hot liquor is then carried in a large tub, called the _reservoir_, and placed on the edge of the boiler. As soon as the insoluble salts, which the solution contains, are deposited there, which takes place immediately, the liquor is drawn off clear by cocks, which are adapted to the tubs, and received into the boiler, previously cleaned. The deposite obtained in the boiling, is washed with a certain quantity of the solution, which becomes clear, and is then mixed with the preceding liquor. From what has been said, the liquor must contain a great quantity of nitrate of potassa, a small quantity of the salts of lime and magnesia, and all the marine salt contained in the plaster. It is frequently the case, that the liquor contains muriate of potassa, and a small quantity of sulphate of lime. It is, therefore, submitted again to evaporation. When it is at the forty-second degree of concentration, some part of the marine salt separates, which rises to the surface, and is taken off, and drained through an osier basket placed over the boiler. The solution being concentrated to the forty-fifth degree of the hydrometer, it is put into copper vessels, in which, by cooling, it crystallizes. The salt is then separated from the mother water, drained and coarsely bruised, and afterwards washed in a certain quantity of the _first boiling_. It is now in a state to be delivered to the central administration, under the name of crude saltpetre, or saltpetre of the first boiling.