Mr. Maffey on Sahpttrt. 22? 
is impoffible to forefee. We can only, therefore, 
lay down fuch rules as may enable the faltpetre 
maker to judge, what quantity of pot-afh he ought 
to employ, according to the (late in which he 
finds thefe earths. 
For this purpofe, let him diffolve one part of 
pot-afh in two parts of water, and either filter 
the liquor, or let it ftand till it is clear, and 
fet this folution afide. 
Let him then elixiviate fome very pure nitrous 
earth in three or four times its weight of water, 
and fet this lixivium likewife afide ; and if, upon 
dropping fome of the folution of pot afh into 
a glafs of his ley, he finds the latter grows 
whitifh or muddy, he may be affured that the 
precipitation of the earth is not completed, and 
confequently, that a fufncient quantity of pot- 
afli has not been employed. If, on the other 
hand, the ley remains untroubled, let him drop 
in fome of the lixivium drawn from the nitrous 
earth, upon which, if the pot-afh is in too great 
a quantity, it will likewife turn muddy. Upon 
the whole, if, in neither of thefe cafes, the liquor 
is difturbed, we may be allured the pot-afh has 
been employed in a juft proportion. 
In general it is better to ufe too little than too 
much pot-afh. When too little is employed, 
it is true, part of the nitre, with an earthy bafis, 
will not be decompofed, but it will be found 
in the mother-water after cryftalllzation j and 
when 
