1. The finest variety, which is also the rarest, is of a delicate azure blue colour by reflected light, and of a faint yellowish 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 counter part in the mineral kingdom in some of the finer semiopals, which approach to the precious varieties.
2. Another variety of tabashecr 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 semiopals.
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 per fectly opaque.
Although these are the forms in which tabasheer generally occurs, yet several peculiarities of struc ture present themselves in the examination of nu merous specimens. In some 1 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 in volved in some uncertainty. That which Dr. Rus sell brought from India in 1790, and which is simi lar to that sent by Mr. Swinton, consisted accord ing to Mr. Smithson, of pure silex; but Fourcroy and Vauquelin* having examined a portion of what Baron Humboldt brought from South Ame rica in 1804, found it to consist of seventy parts of silex and thirty of potash. Dr. Turner,t who at Dr. Brewster's request, made a new analysis of the Indian tabasheer, found it to consist entirely of si lica, with a minute quantity of lime and vegetable matter.
Its specific gravity varies from 2.060 to 2.190. The translucent variety of tabasheer loses 3.84 per cent. and the transparent variety 4.58 by a red heat, losses which it does not recover by exposure to the air.
It feels gritty in the mouth like magnesia, with a slightly nauseous taste. It dissolves readily in a so lution of pure potash, even after being heated to redness.
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 tabashcer ex ceeds in weight the tabasheer itself, and the space occupied by the pores is to that occupied by the solid particles nearly as 21 to 1.
The chalky tabasheer, which does not become transparent by the absorption either of oil of cassia or water, readily imbibes the fat oils, and with oil of beech-nut it becomes as transparent as glass, but it requires a considerable time to displace the air from its pores. These results are perfectly analo gous to those which we obtain with hydrophanous opal, and I have also succeeded in giving transpa rency to the chalky silex from the Giant's Cause way, by long immersion in oil of beech nut.
If, instead of immersing the tabasheer in water, we place a small drop upon the most transparent variety, the drop is instantly absorbed, but the spot which it occupies becomes as white and opaque as if it had been covered with white lead. This ex traordinary property, which is not possessed by any of the siliceous minerals, will be explained when we have treated of the optical properties of this sub stance.
The opaque tabasheer, which has become trans parent by absorbing oil, exhibits a very curious phenomenon by change of temperature. If it is laid upon a piece of cold lead, it becomes suddenly opaque, and if it is restored to a warmer situation, its transparency as suddenly returns. These effects obviously arise from the great expansion and con traction of oil by heat. When the oil retreats from the surface of the specimen, the mutual attraction of its own particles accumulates them in one place, instead of permitting them to remain in a state of contraction in separate pores, as might have been expected. When the greater part of the oil has been expelled from these specimens by heat, the ta basheer exhibits a beautiful veined structure, the veins being sometimes parallel, as in the onyx, and sometimes curved, as in the agate. This effect arises from the different degrees of porosity in the different veins, in virtue of which some of them ab sorb more oil than others. The limits of each vein are thus rendered visible in the very same man ner as the veins of burned chalcedony, which has absorbed oil from the lapidary's wheel, may be dis played in all their beautiful inflexions, although in its natural and transparent state it did not exhibit the slightest trace of such a structure. It is from the same property of some of the amorphous sili ceous minerals that the lapidary is able to develope, and to colour, the veins of particular agates, and that the artist can execute the finest drawings, which actually lie beneath the surface of certain po rous specimens of chalcedony.