This
tabular statement of specific gravities would be incomplete without
reference to the change which many zircons undergo when strongly
heated—most coloured zircons thus lose or change their original hue.
Some become nearly or quite colourless, others from a brown or reddish
hue change to a dull green. Now it is found that zircons naturally of
high density are not altered in this character by heating, but, on the
other hand, that specimens of low density usually contract considerably
after having been raised to a high temperature. Thus a green stone,
having the low density of 4.0 acquired a density of 4.31 after it had
been strongly heated ; while a gold-coloured specimen was raised from
4.375 to 4.657. Yet so long ago as 1875 I described a low density dull
green zircon which suffered no condensation by being heated but
retained its original density of 4.02. The recent researches of Dr. S.
Stevanovic and of Mr. L. J. Spencer point to the existence of three
modifications of zircon-material. One of these has a permanent density
of 4 ; another has the same density, which, however, can be raised by
heating ; a third modification has the permanent density of 4.7. All
the different densities and changes of density which have been observed
can be explained on this hypothesis.
Several
curious phenomena are presented by certain varieties of zircon. For
example, all the bright green stones of low density as well as some
others of ill-defined greyish yellow and greenish yellow
