148 Dr Brewster on a Singular Property of Tahasheer. 
The first person that examined the properties of this sub- 
stance was Mr Macie^' (now Mr Smithson), who analysed a 
portion of the Tabasheer from Hyderabad, which Dr Russell-)' 
had the preceding year presented to the Royal Society. From 
its indestructibility by fire ; — ^its total resistance to acids ;• — its 
uniting by fusion with alkalis in certain proportions into a white 
opaque mass, in others into a transparent permanent glass, and 
its being again separable from these compounds entirely un- 
changed by acids,” he considers it “as perfectly identical with 
common siliceous earths 
In the year 1804, Messrs Humboldt and Bonpland brought 
with them from America some specimens of Tabasheer, called 
Guaduas butter by the Creoles, taken from the bamboos which 
grow to the west of Pinchincha in the Cordilleras of the Andes ij;,. 
These specimens were analysed in 1805, by Messrs Fourcroy 
and Vauquelin §, who found tliem to be different from the 
Tabasheers of Asia. Instead of being wholly composed of silex, 
they contained only 70 per cent, of this earth, and SO per cent* 
of potash, lime and water. 
The Tabasheer which I employed in my experiments, was sent 
from Nagpore by Dr Moore to Dr Alexander Kennedy, who was 
so kind as to favour me with a considerable portion of it. It had the 
same general chemical characters as the Tabasheer from Hydera- 
bad, which was used by Mr Smithson, the same specific gravity 
nearly, and the same external appearance ; so that I have no 
hesitation in considering it as also composed priiicipally of silex. 
When the semi-transparent specimens of this substance are 
immersed in water, they imbibe it with great rapidity, emit- 
ting numerous bubbles of air. The transparency increases 
whenever the air has been discharged, and after a few minutes 
the water pervades, and renders transparent the whole mass. 
If a small portion of w'ater, on the contrary, is laid upon the 
Tabasheer when dry, instead of adding to its transparency as 
might halve been expected, it actually renders it as opaque and 
white as chalk ; and, from the same cause, the Tabasheer which 
has been saturated with water becomes opaque, as the water eva^- 
® See Philosophical Transactions^ 1791, p. 368. 
*)• See Philosophical Transactions^ 1790, p, 273. 
J Humboldt’s Personal Narrative^ vol, i. Introd. p. xiii. Note® 
§ Mcawires de V Institute tom. vi. p. 382, 
