and physical properties of tabasheer. 291 
the mass. In both these cases the penumbra, which separated 
the opaque and transparent portions, had a ragged or branch- 
ing appearance when seen by a microscope, as if the oil had 
been shooting into crystals during the progress of the opacity, 
and as if these crystals had been dissolving during its retreat. 
Upon examining the appearance of the tabasheer when it 
had the colour of red brick, after the discharge of a portion 
of its oil, I was surprised to observe a beautiful veined struc- 
ture, exactly like that of the agate, the veins being sometimes 
parallel, sometimes inflected, and sometimes curved. In 
some specimens the veins were alternately opaque and trans- 
lucent, and in others red and white, as if one set of strata 
had a greater capacity for the oil than the rest. This effect 
is almost universal ; but as soon as the oil is completely 
discharged, the veined structure entirely disappears, and the 
whole mass assumes the homogeneous appearance of chalk. 
In order to observe the circumstances under which the 
chalky tabasheer became transparent by the absorption of 
oils, I cut four plates out of the same piece, and immersed 
them separately in oil of cassia, alcohol, water, and oil of 
beech nut. The plates that had absorbed the three first of 
these fluids remained quite opaque, but the plate that was 
placed in the oil of beech nut gradually acquired a translu- 
cency by the rapid extrication of air. After a certain time 
it appeared to be covered with scratches and small opaque 
portions ; but these appearances, which arose from remaining 
vesicles of air, vanished by degrees. By the application of a 
microscope, I observed the air form itself into globules in 
the interior of the tabasheer, which slowly advanced to the 
edge of it, and. at last escaped into the oil. After a lapse of 
