[ 10. J 



and abftrading the marine fait, which being cryftallized by eva- 

 poration during the procefs, falls to the bottom ; let him filter it 

 through canvas made in the form of a jelly-bag, leaving it tO' 

 eryftallize (after each elixation) in leaden or copper veffels, ex- 

 pofed to a free circulation of air in a dry fituation, and not in a 

 cold cellar, which is frequently, though erroneoufly, pradifed, 

 with this palpable difadvantage, that fudden refrigeration forming, 

 the nitrous cryftals before all the common fait has been preci- 

 pitated, a part of it enters into their compoiition ; they are alfcr 

 of a lefs fize, and not fo compa£l as when the folution is gradually- 

 eooled. It is cuflomary with powder-makers to prefer the cakes- 

 depofited towards the bottom of the pans in which the folution of 

 falt-petre is fet to eryftallize ; thefe are formed by a congeries of 

 minute cryftals, and are confiderably lefs pure than the larger 

 fhoots, being intimately mixed with whatever heterogeneous matter 

 the folution may retain, which is generally precipitated towards 

 the commencement of cryftallization : I fufpe£t the prediledlion 

 for this kind of nitre has no better foundation than its being, 

 more readily pulverized. The mother-water which oozes^-from- 

 the pans is commonly fprinkled on earth intended for generating 

 falt-petre ; inftead of this, was the refiner to add to the mixture 

 a fmall quantity of wood-afhes, and repeat the operation of 

 extrading, he would find it advantageous : He will alfo fave 

 confidcrably by fubftituting iron boilers and leaden pans to his^ 

 copper ones. 



Charcoal affords few new obfervations. I have tried various 

 kinds, with a fcarce perceptible difference in their effeds, pro- 

 vided they were completely charred and eq^ually well pulverized ; 



however, 



