189 On the method of extracting Saltpetre. [April 



lono-, hv 7 feet brond at the surface, the bottom is formed with a 

 double slope of four inches towards the centre, one from the sides, 

 the other louiiitudin dlv, so that the depth at one end is 16 inches 

 and at the other end 20 inches. 



The water is kept constantly in motion by means of wooden rakes 

 to assist in coolinc, it, and the crystals as they form are raked to one 

 end of the reservoir heaping- them up to drain, and removinir the 

 most elevated parts, as they whiten perceptibly, to be carried to 

 trouiihs or baskets and left to drain. In thus removino^ successive- 

 ly the small crystals a:* they form, the agitation, of the water with 

 rakes must be continued incessantly to prevent "the Saltpetre from 

 formin.i- in hir^^e cakes. — When the water has fallen in temperature 

 to w ithin 4 degrees of the jjlace, or in about six hours all the Salt- 

 petre will have been obtained, and the water will remain collected 

 in the centre and towards one end of the reservoir from whence it 

 is easily rcmovtd. 



On the tredtment of the vmter of crystallization. 

 This water will be left fully saturated with nitre and common salt, 

 with a portion of earthy nitrates which may not be decomposed and 

 ought to be again treated with a small proportion of potass, or pass- 

 ed over ashes, and then evaporated — or this water may be mixed 

 with other Saltpetre water, before the latter has been treated with 

 potass. If it is evaporated separately, the water which again 

 remains may be abandoned or sold. 



In Fiance the earthy substances removed from the bottom of the 

 boiler, is washed to recover the nitre remaining in it, the skum is 

 boiled up in good water for the same purpose, and the refuse salt, 

 which also contains a considerable proportion of Saltpetre is treated 

 as follows. 



* A boiler is to be filled three parts full of water and brought to 

 boil ; pure common salt is then thrown in till the water is fully satu- 

 rated, which will be known by some of the salt falling to the bot- 

 tom, the excess is removed with a scoop. Have in readiness some 

 baskets filled with the refuse salt, one of these baskets, is to be im- 

 merged in the boiling solution^ which cannot dissolve any more 

 common salt, and can only act upon the nitre contained in it, in a 

 ehort time, this basket may be withdrawn, and another put in, and 

 so on till the v/ater in the boiler is fully saturated with nitrate of 

 potass, when it may be drawn off to crystallize. The baskets are 

 put into the boiler by means of ropes, and a pulley fixed to a Crane. 



* I have made trial ot this system of treating the refuse salt, and I find 

 that it will amply re-pay the cost, by the quantity of Saltpetre recovered. 



