iio Mr. Maffey on Saltpetre. 
that is extra&ed from them, which mud not only 
injure the faltpetre, but obftrud its cryftal- 
lization. 
All thefe things being confidered, with the 
pra&ice of the Swedes, and the fuccefs of our 
own experiments, we judge ourfelves authorized 
to advife all thofe who are employed in making 
faltpetre, to place but a few wood-afhes at the 
bottom of their tubs, to ferve by way of filter, 
and to fupply their place with pot-afh in the 
following manner. 
When the tubs are filled with earth, the quan~ 
tity of pot-afh we mean to employ, is to be laid 
upon it, and we are to proceed to elixiviate it 
in the ufual way. The water will immediately 
dificlve the pot-afh, which filtering through the 
earth, will decompofe the nitre with an earthy 
bafis, and convert it into faltpetre, and, if the 
pot-afh has been in a juft proportion, none of 
the former will be left behind. It is not necef- 
fary to lay the pot-afh upon the earth, in all the 
tubs, but only upon thofe in the firft row ; becaufe 
thefe earths being wafhed fuccefiively by three 
waters, the lefs pot-afh will remain in them. 
Nothing can be faid, precifely, of the quantity 
of pot-afh to be laid upon any given quantity of 
earth; this depending upon the condition of 
thefe earths, their richnefs, and the quantity of 
nitre, with an earthy bafis that they contain ; and, 
jn fhort, upon many other circumftances which it 
