ical
uniformity, but also because of the unity which exists between their
physical formation and properties.

The first consideration, therefore, may advisedly be that of their
crystals, since their crystalline structure forms a ready means for the
classification of stones, and indeed for that of a multitudinous variety
of substances.

It is one of the many marvellous phenomena of nature that mineral, as
well as many vegetable and animal substances, on entering into a state
of solidity, take upon themselves a definite form called a crystal.
These crystals build themselves round an axis or axes with wonderful
regularity, and it has been found, speaking broadly, that the same
substance gives the same crystal, no matter how its character may be
altered by colour or other means. Even when mixed with other
crystallisable substances, the resulting crystals may partake of the two
varieties and become a sort of composite, yet to the physicist they are
read like an open book, and when separated by analysis they at once
revert to their original form. On this property the analyst depends
largely for his results, for in such matters as food adulteration, etc.,
the microscope unerringly reveals impurities by means of the crystals
alone, apart from other evidences.

It is most curious, too, to note that no matter how large a crystal may
be, when reduced even to small size it will be found that the crystals
are still of the same shape. If this process is taken still further, and
the substance is ground to the finest impalpable powder, as fine as
floating dust, when placed under the microscope each speck, though
perhaps invisible to the naked eye, will be seen a perfect crystal, of
the identical shape as that from which it came, one so large maybe that
its planes and angles might have been measured and defined by rule and
compass. This shows how impossible it is to alter the shape of a
crystal. We may dissolve it, pour the solution into any shaped vessel or
mould we desire, recrystallise it and obtain a solid sphere, triangle,
square, or any other form; it is also possible, in many cases, to
squeeze the crystal by pressure into a tablet, or any form we choose,
but in each case we have merely altered the _arrangement_ of the
crystals, so as to produce a differently shaped _mass_, the crystals
themselves remaining individually as before. Such can be said to be one
of the laws of crystals, and as it is found that every substance has it