t so able a physiologist as Prof. Cohn
intends to investigate the conditions under which living plants separate
this substance from their tissues. That unicellular algae, like the
Diatomaceae, living in a medium which may contain only one part in 10,000
by weight of dissolved silica, or even less than that amount, should be
able to separate this substance to form their exquisitely ornamented
frustules is one of the most striking facts in natural history, whether
we regard it in its physiological or its chemical aspects.

Sir David Brewster long ago pointed out the remarkable physical
characters presented by the curious product of the vegetable world known
as "tabasheer," though so far as I can find out it has not in recent
years received that attention from physicists which the experiments and
observations of the great Scotch philosopher show it to be worthy of.

Tabasheer seems to stand in the same relation to the mineral kingdom as
do ambers and pearls. It is in fact an _opal_ formed under somewhat
remarkable and anomalous conditions which we are able to study; and in
this aspect I have for some time past been devoting a considerable amount
of attention to the minute structure of the substance by making thin
sections and examining them under the microscope. It may be as well,
perhaps, to give a short sketch of the information upon the subject which
I have up to the present time been able to obtain, and in this way to
call attention to points upon which further research seems to be
necessary.

From time immemorial tabasheer has enjoyed a very high reputation in
Eastern countries as a drug. Its supposed medicinal virtues, like those
of the fossil teeth of China and the belemnites ("thunderbolts") of this
country, seem to have been suggested by the peculiarity of its mode of
occurrence. A knowledge of the substance was introduced into Western
Europe by the Arabian physicians, and the name by which the substance is
generally known is said to be of Arabic origin. Much of the material
which under the name of "tabasheer" finds its way to Syria and Turkey is
said, however, to be fictitious or adulterated.

In 1788 Dr. Patrick Russell, F.R.S., then resident at Vizagapatam, wrote
a letter to Sir Joseph Banks in which he gave an account of all the facts
which he had been able to collect with respect to this curious substance
and its mode of occurrence, and his interesting letter was published in
the Philosophical Transactions