BOOK XII. 



563 



librae of the latter. Shortly afterward the solution will be found to be clear 

 and blue. It is boiled until the waters, which are easily volatile (subtiles), are 

 evaporated, and then the greater part of the salt, after it has settled at the 

 bottom of the pan, is taken out with iron ladles. Then the concentrated 

 solution is transferred to the vat in which rods are placed horizontally and 

 vertically, to which it adheres when cold, and if there be much, it is condensed 

 in 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 being 

 cut out and washed with its own solution, is thrown on to boards that it may 

 drain and dry. The yield of saltpetre will be much or httle in proportion 

 to whether the solution has absorbed much or httle ; when the saltpetre 

 has been obtained from lye, which purifies itself, it is somewhat clear and 

 pure. 



The purest and most transparent, because free from salt, is made if it is 

 drawn off at the thickening stage, according to the following method. There 



A — Caldron. 



B— Large vat into which sand is thrown. C— Plug. D— Tub. 

 E — Vat containing the rods. 



