216 
On the Purification 
raised twelve inches above the ground ; by this means* 
the brink of the crystalizing vessel will be twenty- seven 
inches above the ground* which will render the service of 
it more easy and convenient. 
It appeared to us advantageous* to give the bottom 
of the crystalizing vessel an inclination of four inches 
(in the longitudinal direction only) from the sides to 
the centre. 
The solutions from the cauldron may be emptied se- 
veral times successively into the vessels after having ta- 
ken away the deposition of crystals arising from each so- 
lution. 
Four men seem necessary for the service of the crysta- 
lizing vessel. They must keep the liquor constantly 
stirred* by moving the rakes therein ; they must continu- 
ally bring towards the edges of the vessel the crystals 
which are formed; they must take them out with a skim- 
mer* and carry them to the baskets which are to receive 
them* and in which they are to drain. 
The same men may put the saltpetre into the wooden 
boxes in which the draining is completed* and may after- 
wards carry it into the magazine for purified saltpetre. 
For want of a large vessel for crystalization* a shallow 
cauldron may be made use of. 
On Prying the Saltpetre . 
To render the saltpetre fit to be made use of in the 
preparation of gunpowder* as soon as it is purified* it 
may be dried by either of the two following processes. 
First* by exposing it to the open air, or to the sun* during 
some hours* upon such tables as are used for drying gun- 
powder. Secondly* by putting it into a shallow caul- 
dron* and keeping it* for the space of two hours* in a heat 
of from 40 to 50 degrees. 
