and physical properties of tabasheer. 285 
By slightly wetting the tabasheer, it loses all its transpa- 
rency, and assumes the appearance of a piece of chalk ; but 
if we immerse it in water a great quantity of gas is disen- 
gaged, the edges become more transparent than before, and 
a small white ball appears in the centre, which gradually 
diminishes till the transparency has extended itself through- 
out the whole mass. The same effect takes place with the 
second kind of tabasheer ; but the third kind, though it dis- 
engages gas like the rest, never loses its chalky opacity. 
The property of becoming more transparent by the expul- 
sion of air and the absorption of water, is one which the 
tabasheer possesses in common with hydrophanous opal; 
but the faculty of retaining a considerable degree of transpa- 
rency when it is dry, and its pores filled with air ; and the 
still more extraordinary faculty of becoming quite opaque by 
the absorption of a small quantity of water, are possessed by 
no other known substance in nature, and indicate a singula- 
rity of structure which it becomes interesting to investigate. 
When the pores of hydrophane are filled with air, the differ- 
ence between the refractive power of the air and the solid 
substance is so great, that the light is scattered in every 
direction by refraction, and the mass is consequently white 
and opaque. As the tabasheer disengages a much greater 
quantity of air than the hydrophane, its pores must be more 
numerous, and therefore the transmission of the light, so as 
to form a perfect image, indicates either an extreme feeble- 
ness of refractive power, or some singularity in the form and 
construction of the pores themselves. 
In order to determine this, I formed a prism of tabasheer, 
and upon measuring its refractive power, I found it to be 
