Unfortunately, too, it is the
custom of the natives to prepare the substance for the market by an
imperfect calcination, and hitherto I have only been able to study
specimens procured in the markets which have been subjected to this
process. It is obviously desirable, before attempting to interpret the
structures exhibited, under the microscope, to compare the fresh and
uncalcined materials with those that have been more or less altered by
heat.
Tabasheer would seem, from Brewster's experiments, to be a very intimate
admixture of two and a half parts of air with one part of colloidal
silica. The interspaces filled with air appear, at all events, in most
cases, to be so minute that they cannot be detected by the highest powers
of the microscope 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.
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