290 Dr Brewster on the Natural History 



ish hue by transmitted light. It is easily crushed between the 

 fingers, and it has an aerial and unsubstantial texture which 

 we look for in vain in any other solid. It has its counterpart 

 in the mineral kingdom in some of the finer semiopals, which 

 approach to the precious varieties. 



2. Another variety of tabasheer reflects a yellow tint like 

 that of molybdate of lead, and transmits a light of a reddish 



• yellow tinge. It resembles greatly some of the yellow semio- 

 pals. 



3. A third variety is nearly white, with a slight tinge of 

 blue, and is translucent at the edge like cacholong. 



4. A fourth variety resembles chalk, and is perfectly 

 opaque. 



Although these are the forms in which tabasheer generally 

 occurs, yet several peculiarities of structure present themselves 

 in the examination of numerous specimens. In some I have 

 observed a layer exactly like jasper, and in one specimen the 

 surface is covered with a brilliant enamel possessing all the 

 lustre of pure quartz. 



The chemical composition of tabasheer is still involved in 

 some uncertainty. That which Dr Russell brought from In- 

 dia in 1790, and which is similar to what is now on the table, 

 consisted, according to Mr Smithson, of pure silex ; but 

 Fourcroy and Vauquelin, having examined a portion of what 

 Baron Humboldt brought from South America in 1804, found 

 it to consist of seventy parts of silex and thirty of potash.* 



When we plunge any of the varieties of tabasheer in water, 

 an effervescence takes place, owing to the rapid escape of air 

 from its pores ; and when this has ceased, the transparent and 

 translucent varieties have their transparency and translucency 

 greatly increased, but the chalky kind retains its opacity 

 The quantity of water imbibed by the tabasheer exceeds in 

 weight the tabasheer itself, and the space occupied by the pores 

 ife to that occupied by the solid particles nearly as 2 J to 1. 



The chalky tabasheer which does not become transparent 

 by the absorption either of oil of cassia or water, readily im- 

 bibes the fat oils, and with oil of beech-nut it becomes as 

 transparent as glass, but it requires a considerable time to dis- 



• An analysis of Tabasheer by Dr Turner will be found in a subsequent 

 article of this Number. 



