326 THE DIAMOND
liquefying,
and in the presence of oxygen at a high temperature combines with it to
form a gas, the union being accompanied by light and heat. The problem
has been, how to separate it from its affinities and establish it as a
single element in the stable crystalline state.
Probably
the first definite theory on record of the origin of the diamond is
that of Sir David Brewster, who believed that the diamond was at one
time viscous like a resin, and that its formation came from the vital
processes of plants, as tabasheer, a form of silica, grows in the stem
of the bamboo. This theory was accepted by later eminent mineralogists
and physicists. Others adopted it with various modifications. Some
thought it a product of the decomposition of extinct plants by which,
through the evaporation of the decomposition products, pure carbon only
was finally left, and that this eventually was transformed from an
amorphous to a crystalline state. This theory assumed that the
processes were evolved at a low temperature, as graphite would result
from high temperature.
Others
thought that heat was necessary, and that small particles of
carbonaceous matter, contained in an igneous rock or taken up from
neighboring sources during the passage through them of a volcanic
magma, crystallized out as diamond as the mass cooled.
Several
thought that large quantities of carbon dioxide in the interior of the
earth were reduced at a high temperature by other metals present, the
pure carbon crystallizing in the process. From the fact that liquid
carbon dioxide is supposed to exist in cavities in some diamonds, one
scientist formed the opinion that liquid carbon dioxide at a high
temperature and under great
