and Properties of Tahasheer. 293 



Hence it appears that the refractive power of tabasheer is ac- 

 tually nearer that of air than that of water. The index of re- 

 fraction 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 measures : 



Tabasheer, 1.1114 Tabasheer, 1.1503 



do. 1.1145 do. 1.1535 



do. 1.1292 ' do. 1.1825 



do. 1.1454 



The specimen of tabasheer which I have described as cover- 

 ed with a brilliant enamel, possesses great hardness ; and from 

 the measure which I have taken of its angle of maximum po- 

 larization, I have no doubt that its refractive power approach- 

 es to that of the semiopals. 



Th^ determination of the low refractive power of tabasheer 

 enables us to give a satisfactory explanation 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, Plate IV. Fig. 3, is a prism or piece of tabasheer, we 

 may suppose one of its pores, highly magnified, to be represented 

 hj abed. This space is filled with air, and when a ray of light 

 MN, enters the separating surface AB at e, and quits it at A, 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 abed, so as not 

 to fill it, but merely to line its circumference with a film which 

 terminates at cc 8 y d. Then the light which passes from wa- 

 ter into air at^j and again from air into water at g, will suf- 

 fer a comparatively great refraction, and will be considerably 

 scattered in all directions. Hence the tabasheer must appear 

 opaque. If we now saturate it with water, so as to fill the 

 pore abed, the refractions aty and g are removed, and the 

 ray ef will pass on to h unobstructed, so as to experience no 

 change of direction, except the small one which takes place at 

 e and h, where it enters and quits the fluid. 



