roscope which I have been able to employ. It is this intimate
admixture of a solid with a gas which probably gives rise to the curious
and anomalous properties exhibited by this singular substance.

The ultra-microscopical vesicles filled with air in all probability give
rise to the opalescence which is so marked a property of the substance.
Their size is such as to scatter and throw back the rays at the blue end
of the spectrum and to transmit those at the red end.

When the vesicles of the substance are filled with Canada balsam, and a
thin slice is cut from it, this opalescence comes out in the most
striking manner. Very thin sections are of a rich orange yellow by
transmitted light, and a delicate blue tint by reflected light. I do not
know of any substance which in such thin films displays such striking
opalescence.

That the excessively low refractive power of tabasheer is connected with
the mechanical admixture of the colloidal silica with air seems to be
proved by the experiments of Brewster, showing that with increase of
density there was an increase in the refractive index from 1.111 in
specimens of the lowest specific gravity to 1.182 in those of the highest
specific gravity. Where the surface was hard and dense, Brewster found
the refractive index to approach that of semi opal. The wonderful thing
is that a substance so full of cavities containing gas should
nevertheless be transparent.

By the kindness of Mr. F. Rutley, F.G.S., I am able to supply a drawing
taken from one of my sections of tabasheer.

The accompanying woodcut gives some idea of the interesting structures
exhibited in some sections of tabasheer, though much of the delicacy and
fidelity of the original drawing has been lost in transferring it to the
wood.

In this particular case, the faint punctation of the surface may possibly
indicate the presence of air vesicles of a size sufficiently great to be
visible under the microscope. But in many other instances I have failed
to detect any such indication, even with much higher powers. The small
ramifying tubules might at first sight be taken for some traces of a
vegetable tissue, but my colleague, Dr. Scott, assures me that they do
not in the least resemble any tissue found in the bamboo. I have myself
no doubt that it is an inorganic structure. It is not improbably
analogous to the peculiar ramifying tubules formed in a solution of water
glass when a crystal of copper sulphate is suspended