187 On the method of extracting Saltpetre. [April 



pan — the produce thus obtained will shew the quantity of alkaline 

 matter, changed with a part colouring and extractive, and a portion 

 of foreign salts. — If this produce is calcined in a crucible, the pure 

 potass alone will remain. 



Mode of conducting the evaporation. 



To proceed with advantage in this operation, it is necessary, first. 

 That the saltpeti-e water should be of suiHcient strength to prevent 

 the unnecessary expenditure of fuel — second. That the water should 

 have been sufHciently treated with potass, to admit of the evaporation 

 being contiinied to the extent required, without thatswelling or over 

 boiling which is hurtful — and lastly. That there should be a suffi- 

 cient quantity of water collected, before the operation is commenced 

 upon, not only to fill the boiler, but to fill it up, as the water eva- • 

 porates, and by continuing this operation without ceasing to ob- 

 tain by one process as much Saltpetre as possible. 



In general it is advisable to stop the evaporation at the point when 

 the nitrate of potass and the water are in equal parts as the operati- 

 on can be better conducted, with less obstruction from the earthy 

 salts, and other refuse more easily and exactly separated, than if 

 pushed further. 



Accordingly it is only by knowing the capacity of the boiler, that 

 a calculation can be made. If for instance the water shews a den- 

 sity of 15 degrees by the Areometre for nitre — which indicates that 

 it contains 0.12 of nitre- and if the boiler contains 1200 Kilo- 

 grammes — then this quantity of saltpetre water, may be considered 

 to consist of 1 020 Kilogrammes of water, and 1 80 Kilogrammes 

 of salts, or about 144 Kilogrammes nitrate of potass, and 

 36 of Muriates, in order therefore to bring the nitre and water to 

 equal parts, 876 Kilogrammes of that salt must be added, this quan- 

 tity of nitre is represented by 7,300 Kilogrammes of saltpetre water 

 at 15 degrees, or 12 per cent. 



Accordingly it is this quantity of 7,300 Kilogrammes equivalent 

 to 68 hectolitres (about 1,743 English gallons) with which it is ne- 

 cessary to fill up the evaporating boiler ; when all this additional 

 water has been evaporated the operation may be brought to a con- 

 clusion. 



Having collected a sufficient quantity of saltpetre water to carry 

 through one operation, the boiler is to be filled, and brought to boil, 

 and replenished gradually as the water evaporates, this is best done 

 by placing a pan, kept constantly full of this water, near the surface 

 of the boiler and by means of a stop cock, to let off' the water gradu- 



