Dr Turner's Chemical Eocamination of Tabasheer. 335 



quantity of cold water, the whole being contained in a close 

 vessel as usual. Upon examination after five days, the cap- 

 sule, instead of 30 grains of the dry salt, was found to. con- 

 tain 12 grains of a solution of it, the greater part of the 

 salt having passed over into the reservoir of water below, 

 to which it had imparted its taste and properties. The habi- 

 tudes of subcarbonate of ammonia in an atmosphere saturated 

 with aqueous vapour are therefore exceedingly different from 

 those of any other salt, as, instead of attracting water or remain- 

 ing unaffected, it is itself attracted by water and dissolved. 



Such are the principal facts which have presented themselves 

 in the investigation of the absorption of vapours by liquids. 



Art. XVII. — Chemical Examination of Tabasheer. By Ed- 

 ward Turner, M. D., F. R. S. E. Professor of Chemistry 

 in the University of London.* Communicated by tlie Author^ 



Having completed an analysis of tabasheer, for a supply of 

 which I am indebted to the kindness of Dr Brewster, I con- 

 ceive that a short account of the chemical examination, as con- 

 nected with the interesting paper lately communicated by that 

 gentleman, will not be unacceptable to the Society. A further 

 appeal to analysis for illustrating the chemical constitution of 

 tabasheer seems the more necessary, since the results of the 

 two analyses of it which have been published differ consider- 

 ably from each other. For according to the experiments of 

 Mr Macie, detailed in the Philosophical Transactions for 1791, 

 p. 368, the tabasheer brought from India by Dr Russell con- 

 sists of pure siliceous earth; while the specimen brought from 

 South America by Messrs Humboldt and Bonpland was found 

 by Fourcroy and Vauquelin to contain only 70 per cent, of 

 silica, the other 30 consisting of potash, lime, water, and a 

 small quantity of vegetable matter.-f- This difference is both 

 considerable and important. The siliceous concretions in the 

 joints of the bamboo mus^ surely have existed in the sap in a 

 state of solution ; and from the large proportion of silica con- 

 tained in the epidermis of the bamboo, it may also be inferred 



* Read at the Royal Society on March 3d. 



t Memoires de I'InstUuL torn. vi. p. 382. (1806.) 



