Tabasheer is a substance found in the cavities of the bamboo, existing originally in the state of a transparent fluid, but gradually indurating into a solid of different degrees of hardness: it consists of 70 silica, + 30 potash and lime. One variety has a milky transparency, transmitting a yellowish, and reflecting a bluish light; another is translucent, and a third opake: the two first varieties become transparent, and evolve air when immersed in water: the third evolves air also, but remains opake. If the first varieties be only slightly wetted they become quite opake. The property of acquiring transparency by the evolution of air from, and the absorption of water by its pores, belongs also to the hydrophanous opal; but the faculty of becoming opake by a small quantity, and transparent by a larger, of water, shows a singularity of structure in tabasheer. As the tabasheer disengages more air than hydrophane, its pores must be more numerous; and therefore the transmission of light, so as to form a perfect image, indicates either a very feeble refractive power or some peculiarity in the construction of its pores. To determine this, Dr. Brewster formed a prism of tabasheer with an angle of 34° 15', and upon measuring its refractive power found it very low, though various in different specimens, the index of refraction varying from 1·11 to 1·18, that of water being 1·33, of flint-glass 1·60, of sulphur 2·11, of phosphorus 2·22, and of the diamond 2·47. So that tabasheer has a lower refractive power than any other solid or liquid, and holds an intermediate place between water and the gases. Dr. Brewster then gives a formula for computing the absolute refractive power of bodies, and a table of results, from which it appears that, in this respect, the refractive power of tabasheer is so low as to be separated by a considerable interval from all other bodies. The author next proceeds to detail a variety of experiments upon the absorbent powers of the different kinds of tabasheer, in respect to several liquids, and the corresponding effects upon its optical properties and specific gravity, and concludes with observations on the cause of the paradox exhibited by the transparent tabasheer, in becoming opake by absorbing a small quantity of water, and transparent when the quantity is increased.
On the optical and physical properties of tabasheer. By David Brewster, LL. D. F. R. S. Lond. and Edin. In a letter to the Right Hon. Sir Joseph Banks, Bart. G. C. B. P. R. S. &c