t--or, in other words, to exhibit the phenomenon of
_opalescence_. When tabasheer is slightly wetted, it becomes white and
opaque; but when thoroughly saturated with water, perfectly transparent.

By preparing prisms of different varieties of tabasheer, Brewster
proceeded to determine its refractive index, arriving at the remarkable
result that tabasheer "has a lower index of refraction than any other
known solid or liquid, and that it actually holds an intermediate place
between water and gaseous bodies!" This excessively low refractive power
Brewster believes to afford a complete explanation of the extraordinary
behavior exhibited by tabasheer when wholly or partially saturated with
fluids. A number of interesting experiments were performed by saturating
the tabasheer with oils of different refractive powers, and by heating it
in various ways and under different conditions, and also by introducing
carbonaceous matter into the minute pores of the substance by setting
fire to paper in which fragments were wrapped.

The mean of experiments undertaken by Mr. James Jardine, on behalf of
Brewster, for determining the specific gravity of tabasheer, gave as a
result 2.235. From these experiments Brewster concluded that the space
occupied by the pores of the tabasheer is about two and a half times as
great as that of the colloid silica itself!

From this time forward Brewster seems to have manifested the keenest
interest in all questions connected with the origin and history of a
substance possessing such singular physical properties. By the aid of Mr.
Swinton, secretary to the government at Calcutta, he formed a large and
interesting collection of all the different varieties of tabasheer from
various parts of India. He also obtained specimens of the bamboo with the
tabasheer _in situ_. In 1828 he published an interesting paper on "The
Natural History and Properties of Tabasheer" (_Edinburgh Journal of
Science_, vol. viii., 1828, p. 288), in which he discussed many of the
important problems connected with the origin of the substance. From his
inquiries and observations, Brewster was led to conclude that tabasheer
was only produced in those joints of bamboos which are in an injured,
unhealthy, or malformed condition, and that the siliceous fluid only
finds its way into the hollow spaces between the joints of the stem when
the membrane lining the cavities is destroyed or rent by disease.

Prof. Edward Turner, of the University