1836.] 



Chemical Analysis of TahasJieer. 



491 



It is sufficiently known that tabasheer is a concretion met with occa- 

 sional! y in the joints of the bamboo; that it has been long employed 

 in medicine, in Hindostan and the East ; that it is very much esteem- 

 ed; and, that it sells at a considerable price. The first good descrip- 

 tion of it was drawn up by Dr. Russel, and published in the " Philoso- 

 phical Transactions" for 1790, p. 273. The specimen, laid before the 

 Royal Society, by Dr. Russel, was put into the hands of Mr. Smithson 

 for chemical examination. A very minute, accurate, and complete set 

 of experiments, by this acute and accomplished philosopher, was pub- 

 lished in the "Philosophical Transactions" for 1791, p. 368, from 

 which it appeared, that the tabasheer was composed of silica nearly in 

 a state of purity. 



In the year 1805, a specimen of tabasheer, from Pervr, was put into 

 the hands of Fourcroy and Vauquelin, by Humboldt and B on plan d. 

 These chemists subjected it to analysis, extracted from it 70 per cent, 

 of silica, together with a little lime, and concluded (though it is not 

 easy to see the evidence), that the tabasheer, which they examined, 

 w r as a compound of 70 parts of silica, and 30 parts of potash. But 

 under the potash were included the vegetable matter which they show- 

 ed it to contain, and also, the water, the amount of which, they seem 

 not to have thought of determining. 



In 1819, a curious paper on the optical properties of tabasheer, was 

 published in the " Philosophical Transactions" by Dr. Brewster. An 

 abstract of this paper, together with several particulars, relative to the 

 history and formation of the tabasheer, was inserted in the eighth vo- 

 lume, of Dr. Brewster's " Journal of Science ;" and in the same vo- 

 lume, we have a chemical examination of the tabasheer, by Dr. Turner. 

 This analysis agrees very nearly with that of Mr. Smithson, and ren- 

 ders the accuracy of the statement of the great quantity of potash, 

 announced by Fourcroy and Vauquelin, rather doubtful. 



1. The tabasheer which I examined, was a very beautiful looking 

 substance, in small irregular fragments of a blueish white colour and 

 pearly lustre, not unlike chalcedony in appearance, but much softer. 

 For it was incapable of scratching calcarious spar, and only slightly 

 scratched sulphate of lime. When put into water, it gives out a great 

 deal of air with a kind of crackling noise, and imbibes a great dea* 

 of water. 



I found its specific gravity, (taken without allowing time for the 

 internal air to escape), 1-9238. But, when by means of heat all the air 

 bubbles had been driven off, the specific gravity was as high as 2-0824. 



2. When ignited, it lost 4*87 per cent, of its weight. This loss 

 consisted chiefly of water; but not entirely, for the tabasheer exhaled 

 a peculiar odour, and, showed evidently, the existence of a small quan- 

 tity of vegetable matter in it. 



