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16
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PRECIOUS STONES.
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conform
to the Figures of the Cavity that contain it, and which in this case
has not so much the Nature of a Womb, as of a Mold. And so we see that
Salt-Petre, and divers other Salts, if the Water, they were dissolv'd
in, be much too far boyl'd away before they are suffer'd to shoot,
will, if the Liquor fill the Glass, sometimes coagulate into a Mass,
fashion'd like the inside of the containing Vessel, or if a pretty
quantity of Liquor remain after the coagulation, that part of the
nitrous Mass, that was reduc'd to be concreted next the Glass, will
have the shape of the Internal surface of it, whatever that be; but
those Christals that are contiguous to the remaining Liquor, having a
Fluid Ambient to shoot in, will have those parts of their Bodies, that
are contiguous to the Liquor, curiously form'd into such Prismatical
shapes as are proper to Nitre." It may be remarked in passing that when
Boyle speaks expressly of "determinate " shapes and the
prismatical shapes proper to nitre, it seems as if he had recognised
the fact that a given compound crystallises in one definite shape,
although this has been denied.
As
previously pointed out, Bischof in 1854 placed before geologists and
physicists a splendid series of observations from which he deduced the
very inportant influence of water in the formation of many compounds
commonly supposed to result from the action of dry heat.
The
view suggested by J. G. Goodchild in giving the present classification,
was one that had for some years previously been taught by him ; for a
full explanation of it the reader must be referred to the paper
mentioned above ; in outline it is as follows: If a large intrusive
sheet of rock be followed over a considerable area of country, its
character to the eye is seen to change but little, and on
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