The absorptive power of tabasheer is not confined to fluids. It draws into its pores solid bodies in a minute state of subdivision. If we wrap a piece of it in a bit of paper, and burn the paper, the taba sheer will come out of it of a glossy black colour, transmitting only red light, like a piece of smoked glass. By repeating this operation twice or thrice, it becomes so deeply black as not to admit a ray of the meridian sun. By exposing the specimen to a white heat the black matter is discharged, and the tabasheer is restored to its former appearance and properties. When the blackened tabasheer is plunged in water it disengages the included air, but with less rapidity than before, because there is less air to disengage; and when it is broken and pounded, its fracture and its powder are black. If the black matter has not insinuated itself copiously into the heart of the specimen, this portion is of a bluish slate-colour. When slightly wetted in this place it becomes white, and when saturated with water it becomes jet black. This, however, is an illusion; for though it does appear absolutely black, yet it is, in reality, made translucent by the absorp tion of the water. This translucency allows the white light, which the nucleus formerly reflected, to pass on to the black coating, where it is absorbed, —an effect analogous to what takes place in a black inkstand, in which it is impossible to distinguish ink from water by looking at the surface of the fluid.
One of the most remarkable properties of the ta basheer is its low refractive power, which is lower than that of any other body, whether solid or fluid, as will be seen from the following table:— Hence it appears, that the refractive power of ta basheer is actually nearer that of air than that of water. The index of refraction given above is the
lowest that I have obtained; but specimens of greater specific gravity have higher refractive powers, as will be seen from the following mea sures:— The specimen of tabasheer which I have described as covered with a brilliant enamel, possesses great hardness; and from the measure which I have taken of its angle of maximum polarization, I have no doubt that its refractive power approaches to that of the semiopals.
The determination of the low refractive power of tabasheer enables us to give a satisfactory explana tion of the curious fact already mentioned, that a small drop of water produces white opacity, while a greater quantity renders it perfectly transparent.
If ABC is a prism or piece of tabasheer, we may suppose one of its pores, highly magni fied, to be repre sented by a b c d.
This space is fill ed with air, and when a ray of light MN, enters the separating surface AB at e, and quits it at h, it. suffers so little refraction, that the tabasheer allows us to see objects distinctly through it. Let us now suppose that a small quantity of water is introduced into the pore a b c d, so as not to fill it, but merely to line its circum ference with a film which terminates at d $ yd. Then the light which passes from water into air atf, and again from air into water at g, will suffer a comparatively great refraction, and will be con siderably scattered in all directions. Hence the tabasheer must appear opaque. If we now saturate it with water, so as to fill the pore a b c d, the re fractions at f and g arc removed, and the ray c f will pass on to h unobstructed, so as to experience no change of direction, except the small one which