BOOK XII.

563

librae of the latter. Shortly afterward the solution will be found to be clearand blue. It is boiled until the waters, which are easily volatile {subtiles), areevaporated, and then the greater part of the salt, after it has settled at thebottom of the pan, is taken out with iron ladles. Then the concentratedsolution is transferred to the vat in which rods are placed horizontally andvertically, to which it adheres when cold, and if there be much, it is condensedin three or four days into saltpetre. Then the solution which has not con-gealed, is poured out and put on one side or re-boiled. The saltpetre beingcut out and washed with its own solution, is thrown on to boards that it maydrain and dry. The yield of saltpetre will be much or little in proportionto whether the solution has absorbed much or little ; when the saltpetrehas been obtained from lye, which purifies itself, it is somewhat clear andpure.

The purest and most transparent, because free from salt, is made if it isdrawn off at the thickening stage, according to the following method. There

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