%g6 Dr. Brewster on the optical 
degree of porosity ; and as it makes the pore more extensive 
in the transparent than in the opaque kind, contrary to 
what we should expect from their specific gravities, it seems 
to follow, that the water was not capable of insinuating itself 
into all the pores of the opaque tabasheer. This conclusion 
is rendered more probable, when we consider the extreme 
difficulty with which the oil of beech nut displaces the last 
portions of included air; and it affords a very plausible 
explanation of the fact, that the chalky tabasheer cannot be 
rendered transparent by the absorption of water. 
We are now prepared by the preceding observations, for 
investigating the cause of the remarkable paradox exhibited 
by the transparent tabasheer, in becoming perfectly opaque 
and white, by absorbing a small quantity of water, and per- 
fectly transparent when that quantity is increased. As this 
effect takes place indiscriminately with all fluids, it cannot be 
the result of any chemical action, and therefore its cause 
must be sought for in the changes which the light suffers in 
traversing the vacuities of the tabasheer. 
Let ABC be a prism of this substance, and abed one 
of its pores highly magnified. We know that this pore 
is filled with air ; and 
that when a ray of light a 
MN enters the separat- 
ing surface ab at e, and 
quits it at Ji , it suffers so 
little refraction, and is 
therefore so little scat- 
tered, that the tabasheer 
appears transparent, and c 
allows us to see objects 
