Oil the Purification of Saltpetre. 209 
The composition is as follows: three parts of air- slaked 
lime^ two of wood-ashes, and one of fine sand, or sea- 
coal ashes ; sift these through a fine sieve, and add as 
much linseed-oil as will bring the mixture to a consist- 
ence for working with a painter’s brush. Great care 
must be taken to mix it perfectly : I believe grinding it 
as paint would be an improvement. Two coats are ne- 
cessary ; the first rather thin, the second as thick as can 
conveniently be worked. From the nature of this com- 
position, there is no doubt but it is very durable ; as it 
certainly will improve in hardness by time, and is much 
superior for the purpose to any thing I know of. 
NO. 43. 
Instructions respecting the Purification of Saltpetre , 
drawn up by Order of the Committee of Public Safety 
of Paris; by whose Order also this Process is adopted 
in all the Laboratories of France.* 
THE crude saltpetre is first to be bruised with wood- 
en beaters, that the water, with which it is afterwards to 
be washed may more easily act upon every part of it. 
The saltpetre, thus bruised, is then to be carried to 
proper tubs or vats, in each of which five or six hundred 
pounds may be put. 
Water is to be poured upon the saltpetre, in the pro- 
portion of twenty parts to a hundred, and the mixture is 
to be well stirred. 
It is then to be left to macerate or soak, till the liquor 
will dissolve no more. Six or seven hours are sufficient 
for this first operation; the liquor then indicates from 
twenty- five to thirty-five degrees, by the pese-liqueur . 
* Repertory of Arts vol. 8, p, 199. From the Annates de Chimis. 
VOL. It. C C 
