The earth is dug, and carried to hoppers of a very simple construction, which contain about fifty bushels. Cold water is poured on it for some time, and in a day or two, a solution of the salts runs into troughs placed beneath the hoppers. The lixiviation is continued as long as any strength remains in the earth. The liquor is then put into iron kettles, and heated to ebullition; it is afterwards thrown upon a hopper containing wood ashes, through which it is suffered to filtrate. As the alkaline part of the ashes is discharged before the nitrate passes through, the first runnings of this hopper are thrown back, and after some time, the clear solution of nitrate of potassa runs out, mixed with a white curd, which settles at the bottom of the trough. This clear liquor is boiled to the point of crystallization, then settled for a short time, and put into troughs to crystallize, where it remains twenty-four hours; the crystals are then taken out, and the mother water thrown upon the ash hopper, with the next running of the nitrate of lime. When the quantity of the nitrate of lime is too great for the portion of ashes employed, the workmen say their saltpetre is in the _grease_, and that they do not obtain a due quantity of nitre. If too much ashes are used, they say it is in the _ley_; and when it is left to settle previous to crystallization, a large quantity of salt will be deposited in the settling troughs, which they call _cubic salts_. These salts are again thrown upon the ash-hoppers, and are supposed to assist in precipitating the lime from the nitrate of lime, and in the opinion of the workmen are changed into pure saltpetre. To make a hundred pounds of good saltpetre at the great cave, eighteen bushels of oak ashes are necessary; ten of elm, or two of ashes made by burning the dry wood in hollow trees. The earth in some caves does not require half this quantity of wood ashes to decompose the earthy salts. When wood ashes cannot be obtained in sufficient quantity, they make a lixivium of the earth, and boil it down, which they call _thick stuff_. This is put in casks, and transported to a place where ashes can be had. When dissolved and passed through wood ashes, it is changed, as in the former process, into saltpetre. Having thus given the Doctor's account, let us inquire, in the next place, into the theory of the process.