vats. If when tested it tastes very salty, and at the same time acrid, it is
good ; but, if not, then it is condemned, and it must be made to percolate
again through the same material or through a fresh lot. Even two or three
waters may be made to percolate through the same earth and become full
of saltpetre, but the solutions thus obtained must not be mixed together
unless all have the same taste, which rarely or never happens. The first of
these solutions is poured into the first vat, the next into the second, the third
into the third vat ; the second and third solutions are used instead of plain
water to percolate through fresh material ; the first solution is made in
this manner from both the second and third. As soon as there is an abundance of this solution it is poured into the rectangular copper caldron and
evaporated to one half by boiling ; then it is transferred into a vat covered
with a lid, in which the earthy matter settles to the bottom. When the
solution is clear it is poured back into the same pan, or into another, and
re-boiled. When it bubbles and forms a scum, in order that it should
not run over and that it may be greatly purified, there is poured into it three
or four pounds of lye, made from three parts of oak or similar ash and one of
unslaked lime. But in the water, prior to its being poured in, is dissolved rockalum, in the proportion of one hundred and twenty librae of the former to five
to understand. In this case the saltpetre is melted and the sulphur added and set alight.
Such an addition to saltpetre would no doubt burn brilliantly. The potassium sulphate
formed would possibly settle to the bottom, and if the i' greasy matter " were simply
organic impurities, they might be burned off. This method of refining appears to have
been copied from Biringuccio (x., i), who states it in almost identical terms.
Historical Note.—As mentioned in Note 6 above, it is quite possible that
the Ancients did include efflorescence of walls under nitrum ; but, so far as we are aware,
no specific mention of such an occurrence of nitrum is given, and, as stated before, there
is every reason to believe that all the substances under that term were soda and potash.
Especially the frequent mention of the preparation of nitrum by way of burning, argues
strongly against saltpetre being included, as they would hardly have failed to notice the
decrepitation. Argument has been put forward that Greek fire contained saltpetre, but it
amounts to nothing more than argument, for in those receipts preserved, no salt of any kind
is mentioned. It is most likely that the leprosy of house-walls of the Mosaic code (Leviticus
xiv., 34 to 53) was saltpetre efflorescence. The drastic treatment by way of destruction of
such " unclean " walls and houses, however, is sufficient evidence that this salt was not used.
The first certain mention of saltpetre (sal pétrae) is in Geber. As stated before, the date
of this work is uncertain ; in any event it was probably as early as the 13th Century. He
describes the making of " solvative water " with alum and saltpetre, so there can be no
doubt as to the substance (see Note on p. 460, on nitric acid). There is also a work by a
nebulous Marcus Graecus, where the word sal petrosum is used. And it appears that Roger
Bacon (died 1294) and Albertus Magnus (died 1280) both had access to that work. Bacon
uses the term sal petrae frequently enough, and was the first to describe gunpowder (De
Mirabili Potestate Artis et Naturae 1242). He gives no mention of the method of making his
sal petrae. Agricola uses throughout the Latin text the term halinitrum, a word he appears
to have coined himself. However, he gives its German equivalent in the Interpretatio as
salpeter. The only previous description of the method of making saltpetre, of which we are
aware, is that of Biringuccio (1540), who mentions the boiling of the excrescences from walls,
and also says a good deal about boiling solutions from " nitrous " earth, which may or may not
be of " plantation " origin. He also gives this same method of refining with sulphur. In
any event, this statement by Agricola is the first clear and complete description of the saltpetre " plantations." Saltpetre was in great demand in the Middle Ages for the manufacture
of gunpowder, and the first record of that substance and of explosive weapons necessarily
involves the knowledge of saltpetre. However, authentic mention of such weapons only
begins early in the 14th Century. Among the earliest is an authority to the Council of Twelve
at Florence to appoint persons to make cannon, etc., (1326), references to cannon in the
stores of the Tower of London, 1388, &c.
