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.
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