t4 Rffiiitng of Nitre iy Percolation, Soluiiun, feV. 



The folution is to be poured into a proper veflel, where it depofits by cooling about two- 

 thirds of the faltpetre originally taken. The precipitation begins in about half an hour, 

 and terminates in between four and fix hours. But as it is of importance to obtain the 

 faltpetre in fmall needles, becaufe in this form it is msre eafily dried, it is neceflary to agi- 

 tate the fluid during the whole lime of the cryftallization. A flight motion is communi- 

 cated to this liquid mafs by a kind of rake, in confequence of which the cryftals are de- 

 pofited in very flender needles. 



In proportion as the cryftals fall down, they are fcraped to the borders of the veflel, 

 whence they are taken with a flcimmer, and thrown to drain in baskets placed on trefiels, 

 in fuch a manner that the water which paflcs through may either fall into the cryfliallizing 

 vefl^el, or be received in bafons placed underneath. * 



The faltpetre is afterwards put into wooden vefl'els in the form of a mill-hopper or in- 

 verted pyramid with a double bottom. The upper bottom is placed two inches above the 

 lower on wooden ledges, and has many fmall perforations through which water may pafs 

 to the lower bottom, which likewife afibrds a paflage by one fingle aperture. A refervoir 

 is placed beneath. The cryftallized faltpetre is walhed in thefe veflels with 5 per cent, of 

 water ; which water is afterwards employed in the folution of faltpetre in fubfequent ope- 

 rations. 



The faltpetre, after fuflicient draining, and being dried by expofure to the air upon 

 tables for feveral hours, may then be employed in the manufacture of gunpowder. 



But when it is required to ufe the faltpetre in the fpeedy and immediate manufaiflure of 

 gunpowder, it mud be dried much more ftrongly. This may be efl^eded in a (love, or 

 more fimply by heating it in a flat metallic veflTel. For this purpofe the faltpetre is to be 

 put into the Teflel to the depth of five or fix inches, and heated to 40 or 50 degrees of the 

 thermometer (or about 135 of Fahrenheit). The faltpetre is to be flirred for two or three 

 hours, and dried fo much that, when ftrongly prefled in the hand, it fliall acquire no confif- 

 tence, nor adhere together, but refemble a very fine dry fand. This degree of drynefs 

 is not required when the powder is made by pounding. 



From ^hefe circumflances we find, that two faline liquids remain after the operation, 

 (1) the water from the wafliing ; and (2) that from the cryftallizing veflels. 



We have already remarked, that the wafliing of the faltpetre is performed in three fuc- 

 ceflivc operations, in which, upon the whole, the quantity of fluid made ufe of amounts 

 t6 ;^5 per cent, of the weight of the crude faltpetre. Thefe wafliings are eftabliflied on 

 the principle, that col3 water diflblves the muriates of foda, and the earthy nitrates and 

 muriates, together with the colouring principle, but fcarcely attacks the nitrate of potafli. 



The water of thefe three wafhings therefore contains the muriate of foda, the earthy 

 falts, the colouring principle, and a fmall quantity of nitrate of potafli, the amount of which 

 is in proportion to that of the muriate of foda, which determines its folution. 



The water of the cryllallizing veflels contains a portion of the muriates of foda, and of 

 the earthy falts which efcaped the operation of wafliing, and a quantity of nitrate of 

 potafli, which is more confiderable than that of the former folution. 



The waters made ufe of at the end of the operation, to whiten and wafli the cryftals 

 depofited in the pyramidal veflel, contain nothing but a fmall quantity of nitrate of potafli. 



Thefe 



