of Saltpetre. 
conduct it into a reservoir placed at the end of the row of 
tubs. 
These twenty tubs may be disposed in two parallel 
lines. The planes upon which they are fixed may be in- 
clined towards each other ; so that their union may form 
the gutter or channel which is to conduct into the common 
reservoir the water that runs off. 
These tubs are to have an aperture at the distance of 
two fingers’ breadth from the bottom; which aperture 
(besides the stopper which closes it) must have a grated 
or perforated cover. 
Four men may be allotted to the washing of the salt 
petre : they should also have the charge of carrying the 
saltpetre from the magazine to the tubs, and from the 
tubs to the cauldron. 
It is hardly necessary to mention, that the tubs should 
be separate from each other, and disposed in such a man- 
ner that they may be easily served. 
On the Cauldron . 
A conical cauldron, five feet broad, and four feet deep, 
will supply three operations in the day ; and consequent* 
ly will suffice for the purification of fifteen thousand 
pounds of saltpetre. 
One man is sufficient for the service of the cauldron 
On the Vessel for Crystalization . 
The crystalizing vessel should be made of lead, or of 
copper, and should be placed as near the cauldron as 
possible. 
It should be fifteen inches in depth, ten feet in length, 
and eight in breadth. 
It should be fixed upon very solid ground, in such a 
manner that every point of the bottom may be supported. 
The stone or brick work on which it is placed, should be 
