Dr Brewster on the Natural History 



' fXhe absorptive power of tabasheer is not confined to fluids. 

 It draws into its pores solid bodies in a minute state of subdi- 

 vision. If we wrap a piece of it in a bit of paper, and burn 

 the paper, the tabasheer 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 appear- 

 ances 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 copious- 

 ly into the heart of the specimen, this portion is of a bluish slate- 

 colour. When slightly wetted in this place it becomes wfiiie, 

 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 absorption 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 tabasheer 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 : 



• It is on the same principle that the false gems called Doublets form 

 such admirable imitations of the precious stones. No light is allowed to 

 reach the eye except that which is reflected from, or transmitted through 

 an interposed coloured film ; and hence we think that we are viewing the 

 finest gem when we are only looking through a piece of glass. Having 

 had occasion to examine a number of emeralds, which a skilfUl jeweller 

 put into my hands, he pointed out one superior to all the rest; but upon 

 exposing it to polarized light I found it to be a doublet. It is owing 

 to the same cause that octohedriie appears almost black when lying upon 

 its matrix. Its refractive power, which is equal to that of diamond, bends 

 the incident light so much that it cannot escape from the opposite face of 

 the prism : But when the light falls upon a parallel plate of the mineral, 

 its transparency immediately appears. 



