2t8 
On the Purification of Saltpetre . 
parts of fresh water to a hundred, anti then to carry it to 
the ciystalizing vessel. It was supposed, that by this 
means, the washings with cold water might be omitted, 
or considerably diminished. But, a continued boiling, 
kept up for four or five hours, in order to separate the 
sea- salt, is attended with a great waste of time, of fuel, 
and of saltpetre ; and the washings are still indispensably 
necessary, both to take away the colouring matter, and to 
extract the last portions of sea-salt. 
3. It may be supposed, that it would perhaps be pos- 
sible to diminish the quantity of water used in washing ; 
but we must observe, that it is to be feared, that when 
the saltpetre contains a great quantity of sea- salt, the pu- 
rification of it would not be complete, if a less quantity 
of water were made use of than that we have prescribed. 
4. One might perhaps be tempted to diminish the quan- 
tity of water made use of in the solution; but we are con- 
vinced, by repeated experiments, that the proportion we 
have pointed out is the most proper: if it is augmented, 
the saltpetre remains dissolved in the liquor ; if it is de- 
creased, it congeals or precipitates itself in a mass. We 
found, by observation, that the degree of saturation, most 
proper for our operations, was between the sixty-sixth 
and sixty-eighth degree of the pese-liqueur. 
5. It might also be thought, that it would be more sim- 
ple, and more economical, to treat the solutions of crude 
saltpetre with potash ; but it is to be feared, that by so 
doing, a part of this alkali might have the effect of de- 
composing the muriate of soda, and converting it into 
muriate of potash ; and it must be observed, that the last- 
mentioned salt is by no means proper for decomposing 
earthy nitrates, whatever some able chemists may have 
said of it. 
It therefore appears more proper, not to treat the mo- 
ther-waters, nor to make any use of potash, till all the 
sea-salt has been separated by evaporation. 
