Royal Society. 67 



but generally in the form of a hard concretion. Some specimens 

 of it are transparent, and resemble very much small fragments of 

 the artificial pastes made in imitation of opal ; others are exactly 

 like chalk ; while a third kind is of an intermediate character, 

 and has a slight degree of transparency. — The first person who 

 examined the properties of this substance was Mr, Macie (now 

 Mr. Smithson), who analysed a portion of the tahasheer from 

 Hyderabad, which Dr. Russel had the preceding year presented 

 to the Royal Society. " From its indestructibility by fire — its 

 total resistance to acids — its uniting by fusion with alkalies in 

 certain proportions into a white opaque mass ; in others, into a 

 transparent permanent glass; and its being again separable from 

 these compounds entirely unchanged by acids," he considers it 

 as perfectly identical with coinmon siliceous earth. In the year 

 1804, Messrs. Humboldt and Bonpland brought with them from 

 America some specimens of tahasheer called Grtaduas butter by 

 the Creoles, taken from the bamboos which grow to the west of 

 Pinchincha in the Cordilleras of the Andes. These specimens were 

 analysed in 1S05 by Messrs. Fourcrov and Vauquelin, who found 

 them 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 30 per cent, of potash, lime and water. The 

 tahasheer which Dr. Brewster employed in his experiments was 

 sent from Nagpore by Dr. Moore to Dr. Alexander Kennedy, 

 who favoured Dr. B. with a considerable portion of it. It had 

 the same general chemical characters as the tahasheer from Hy- 

 derabad, which was used by Mr. Smithson, the same specific 

 gravity nearly, and the same external appearance ; whence Dr. 

 B. had no hesitation in considering it as also composed of silex. 

 When the semi-transparent specimens of this substance are im- 

 mersed in water, they imbibe it with great rapidity, emitting nu- 

 merous 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 water, on the contrary, is laid upon the tahasheer when 

 dry, instead of adding to its transparency as might have been ex- 

 pected, it renders it as opaque and white as chalk ; and from the 

 same cause, the tahasheer which has been saturated with water 

 becomes opaque as the water evaporates, reaches its maximum 

 degree of opacity, and recovers its semi-transparency when per- 

 fectly dry. The increase of transparency from the absorption of 

 water is an effect easily explained, and is one with which mine- 

 ralogists have been lona familiar in the plia^nomena ot liydro- 

 phanous opal ; but the production of opacity by the absorption 

 of a smaller portion of the same fluid which produces transpa- 

 rency is a fact entirely new, and not easily explicable upon known 



E 2 principles. 



