and physical properties of tabaskeer. 297 
distinctly through it. This effect, which could not take 
place in any other porous substance, arises from the small 
difference between the refractive power of air and taba- 
sheer. 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 contained between abed 
and afiyX. Then the light which was formerly scattered by 
the slight refraction at e and h in passing from tabasheer 
into air, will now be a little less scattered at these points, since 
it passes from tabasheer into water, where the difference of 
refractive power is less ; but in passing from the film of 
water into the air at f, and in entering the water again at g, 
the scattering of the rays will be very considerable, from the 
great difference in the refractive powers of air and water. In 
passing through every pore, therefore, the light is refracted, 
and consequently scattered no less than four times; and hence 
the piece of tabasheer must appear to be opaque. If we now 
saturate it with water, the pore abcdW\W be completely filled; 
the two great refractions which took place at f and g , will no 
longer exist; and the light will suffer only a slight refraction 
at e and h, by which it will be less scattered than when the 
tabasheer was dry. Hence it follows, that when the taba- 
sheer is saturated with water, it ought to transmit the inci- 
dent light freely, and to be more transparent than when 
it is quite dry, a result which is perfectly conformable to 
observation. 
The very singular anomaly presented by the chalky ta- 
basheer in becoming transparent by means of oil of beech 
nut, and not by means of oil of cassia or water, and the 
additional transparency communicated to the transparent 
mdcccxix. Q q 
