an d physical properties of tabasheer. 287 
absolute refractive power of any body, m its index of refrac- 
tion, and S its specific gravity, we shall have R = m f -. 
By means of this formula I have computed the following 
table, for the purpose of showing the peculiar nature of taba- 
sheer, and the general progression in the absolute refractive 
powers of other bodies. 
Table, showing the absolute refractive powers of tabasheer and 
other bodies. 
Tabasheer 976.1* 
Sulphate of barytes 3829.43 
Air, - - Biot 4530. 
* A distinguished member of the Royal Society, whose opinion is entitled to the 
highest consideration, has kindly stated to me, that in estimating the absolute refrac- 
tive power of tabasheer, I should have taken its specific gravity at about 0.66 in place 
of 2.4, which would have given an absolute refractive power not so marvellously dif- 
ferent from that of other bodies. 
My reason for retaining the original number of 976.1, is, that the result obtained by 
using a specific gravity of 0.66 would have been a theoretical, and not an experimental 
result. A body which sinks in water must have a density greater than unity ; and I 
am supported by the high authority of Mr. Cavendish, and also by that of Mr. 
Smithson, in considering the specific gravity of tabasheer as nearly 2.412. Capillary 
spaces which are so large as to contain water, and even the thickest oils and varnishes, 
can never be regarded as forming a part of the body, though, in the present case, it 
is highly probable that these spaces modify the action of the solid particles in the 
manner which I have described. 
Although the idea that tabasheer is quartz expanded till its specific gravity is re- 
duced to about 0.66, holds out a sort of general explanation of its singular properties ; 
yet when we consider that bydropbane, which is also quartz expanded till it is capable 
of absorbing water, is actually opaque ; and when we recollect also that the refractive 
power of bodies does not always diminish when they expand, as is proved by the ex- 
periments of Albert Euler on heated glass, and by the circumstance that the point 
of maximum density in water is not indicated by a maximum of refractive power, its 
optical properties must continue to appear as marvellous as before. 
