132 Dr. Thomas Thomsons 



Article VI. 



Chemical Analysis of Tabasheer. By Thomas Thomson, M.D., 

 F. R. S., L. & E., &c, Regius Professor of Chemistry 

 in the University of Glasgow. 



Having lately received, from Calcutta, a very fine specimen 

 of tabasheer, I was naturally induced to make a few experi- 

 ments on its chemical constitution. 



It is sufficiently known that tabasheer is a concretion met 

 with occasionally in the joints of the bamboo ; that it has 

 been long employed in medicine, in Hindostan and the 

 East ; that it is very much esteemed ; and, that it sells at 

 a considerable price. The first good description of it was 

 drawn up by Dr. Russel, and published in the " Philosophical 

 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 published in the " Phi- 

 losophical 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 1806, a specimen of tabasheer, from Peru, 

 was put into the hands of Fourcroy and Vauquelin, by 

 Humboldt and Bonpland. 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, 

 was a compound of 70 parts of silica, and 30 parts of potash. 

 But under the potash were included the vegetable matter 

 which they showed it to contain, and also, the water, the 

 amount of which, they seem not to have thought of deter- 

 mining. 



In 1819, a curious paper on the optical properties of ta- 

 basheer, 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 volume of Dr. 

 Brewster's " Journal of Science ;" and in the same volume, 

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

 Turner. This analysis agrees very nearly with that of Mr. 



