ORIGIN, PROPERTIES, CLASSIFICATION, ETC. 9
limpid water, and that their color and other essential properties,
were derived from their metallic spirit. Sir John Hill, nearly
a century later, adopted the opinion that they were formed by
the concretion of matter from cohesion or by some kind of
percolation, and that the difference of their constituents and
the manner of coalescence were the causes of their various
qualities, as smoothness, density, transparency, etc. He
further maintained that their constituent matter was a
pellucid, crystalline substance of different degrees of hardness,
and had it been in a perfectly pure state, all precious stones
would have been without color.
Haiiy, the father of modern mineralogy, says most crystals
were formed in water where the constituents, at first separated and suspended, were brought together by force of
mutual attraction ; that is, the particles diffused and floating were brought together by the attraction of cohesion and
precipitated, when they formed a stratum pure and homogeneous. This constitutes the aqueous theory, which has its
opponents.
As most precious stones are transparent or translucent, the
inference has been drawn that their constituents must have
been in the condition of gases or liquids — an opinion sustained
by the discoveries of the microscope, which reveal the fact that
in many different species, water or some other fluid is enclosed
in cavities, often so extremely minute that several millions
occur in a cubic inch. These little cells appear luminous by
reflected light, which gives brilliancy to the gem ; but if the
light be transmitted, they present a dark outline. Some of
these porous crystals not unfrequently burst and fly to pieces
by the application of strong heat, in consequence of the expansion of the enclosed fluid.
Water often forms one of the constituents of rocks, but it is
