Dr Brewster on the General Properties of Tabaslieer. 99 
' In order to observe the nature of the penumbral boundary 
which might be supposed to separate the opaque from the 
transparent partj if they could be both rendered visible in the 
same mass, I saturated the largest piece of tabasheer that I 
had with the coloured oil, and having discharged a good deal 
of it by heat, it became of course opaque. I now held to the 
flame of a candle one of its extremities, which immediately be^ 
came diaphanous, and the transparency gradually pervaded the 
opaque mass. As soon as the opacity disappeared, I allowed 
One extremity of it to cool ; the transparency immediately dis- 
appeared at that part, and the opacity gradually advanced like 
a black cloud, till the whole was overshadowed by the retreat 
of the oil into the interior of the mass. In both these cases the 
penumbra, which separated the opaque and transparent por- 
tions, had a ragged or branching 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 structure, 
exactly like that of the agate, the veins being sometimes paral- 
lel, sometimes inflected, and sometimes curved. In some spe- 
cimens the veins were alternately opaque and translucent, and 
in others red and white, as if one set of strata had a greater 
capacity for the oil than the rest. This ejffect is almost uni- 
versal ; but as soon as the oil is completely discharged, the 
veined structure entirely disappears, and the whole mass as- 
sumes 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 separate- 
ly 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 translucency 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. 
G 2 
