Dr Brewster cm. a Singula^' Pi'operty ofTaba^heer. 14)9 
porates; reaches its maximum degree of opacity ; and recovers its 
semi-transparency when perfectly dry. 
The increase of transparency from the absorption of water, 
is an effect easily explained, and is one with which mineralogists 
have been long familiar in the phenomena of hydrophanous 
epal ; but the production of opacity, by the absorption of a 
smaller portion of the same fluid which produces transparency, 
is a fact entirely new and not easily explicable upon known 
principles. 
After having determined that the white opacity was not the 
result of any chemical change, and must, therefore, have re- 
sulted from optical causes, I attempted to frame some hypothe- 
tical explanation of the phenomenon. In tracing the progress 
of a ray of light through a porous body, having a small quanti- 
ty of water in its pores, and through another which had these 
pores filled with water, I saw that opacity could be produced' in 
the first ease only upon the supposition that the Tabasheer had 
a refractive power considerably lov/er than water. Improbable as 
this supposition was, I immediately formed one of the semi-trans» 
parent specimens into a prism, and found, to my great surprise, 
that the refractive power of Tabasheer was not only lower than 
water, but so much lower, as to be almost intermediate between 
water and the gases. I repeated this experiment with various 
specimens from Nagpore, and also upon one from Hyderabad, 
with which I was favoured by Dr Hope, and which, as it form- 
ed part of the parcel of which Dr Russell had presented a por- 
tion to the Royal Society of London, was the same as that which 
was analysed by Mr Smithson^. 
The following were the results. 
Index of RefractiotSv 
Air, 1.0000-f 
Tabasheer from Hyderabad, yellowish by reflected light, 1.1115 
Tabasheer from Nagpore, i ( 1.1454 
Tabasheer from ditto, harder,... 
Tabasheer from ditto, 
Tabasheer from ditto, very hard, 
Water, 1.3358 
Bluish by reflected 
light, 
1.1503 
1.1535 
U.1825 
* We trust that someof the Members of the Institute of France will be induce/i 
■to measure the refractive power of the Tabasheer brought from Quito by M. Hum- 
