We will now give the process of extracting saltpetre from the rubbish of old buildings, principally plaster, as adopted in France. The lixiviation, in the first place, is performed in the following manner: a certain number of casks or tubs, thirty-six for instance, is placed in three ranges. These tubs are pierced laterally near their bottom, by a hole of about half an inch in diameter, and closed with a cork; they are placed above a trough connected with a reservoir. There is put then into each tub a bucket full of the plaster, previously pounded, which is supported in the casks by cross sticks, a certain distance from the hole, so as not to obstruct the passage of the fluid. After this, a bushel of wood ashes is added, and the tubs are then filled with the plaster. Water is then put into the tubs of the first row, and after some time, the stop cocks are turned; water is then put into the tubs of another row, and the lixiviation is continued until the fluid indicates the zero of Beaumé's areometer. The saline waters, which are thus obtained, are divided into three parts, in proportion to their specific gravity, or quantity of salt they contain. The lixivium, of five degrees of the areometer, is known under the name of _eaux de cuite_. The waters, which are marked between three and five degrees, take the name of _eaux de forte_; and those below three degrees are called _eaux faibles_. According as the waters are weak, they are made to run through another range of tubs, in order to saturate them. When strong and weak solutions are made to pass through the tubs in the same manner, proceeding from the second row to the third, and from the third to the first, the _earths plaster_, &c. being renewed, the lixiviation is not interrupted. The lixiviation, it appears, is thus continued; for we obtain, at the same time, _weak waters_ from the second row, the _strong waters_ from the third, and the _boiling waters_, or those fit to be put into the boilers, from the first.