NITRE NOT A FOSSIL SUBSTANCE.
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is placed between these two bottoms. After the upper part of the ves
sel is full of water, it soaks through, dissolves the nitre in its passage,
and filtres through the straw, leaving the earth behind. The solution
is then drawn off, and the water evaporated to such a degree that the
nitre crystallizes. In this ſtate it is impure, of a brown dirty colour,
and mixed with some sea-salt, which is always found in the materials
affording the nitre. It is separated by crystallization, and by quick
lime and afhes. A brown liquor remains, which does not crystallize,
and when examined, is found to consist chiefly of some lime and
nitrous acid.
Thus it appears from this history of nitre, that it can hardly be
counted with propriety among the fossil productions. It is always
found in the earth, it is true, but always at the furface. There is no
example of its being found at any depth, or the examples are rare, and
the circumstances of them particular. Margraaf and others have found
a little in the wells of great cities; the origin of which can be easily
accounted for. Dr. Withering, in his tranflation of Bergmánn’s Scia
graphia, fays some of his friends affured him they had got nitre from
the waters of some coal-pits. But it is highly probable that it had
been waſhed down into the coal-pits from the superficial foil, by the
rain-water soaking downwards through the earth. It never occurs in
mineral waters. Some authors imagined they found it in these ; but
they mistook another salt for it, and judged of its prefence by the
form of certain crystals which they had obtained; but the form of
crystals is never to be trusted. As it is therefore found only at the
furface of the earth, or not far below it, it has long been a very ge
neral opinion that it is deposited or formed there by the air. Some
have supposed that the air furnifhed at least the principal part of the
nitre, to wit, its acid; which, finding in corrupted animal and vege
table substances the other necessary ingredient, the alkali, united with
it to form a nitre. Others have supposed that the air afforded, not
the nitrous acid, but vitriolic acid. But these opinions have not been