and physical properties of tahasheer. 289 
that of oil of cassia. The oil communicated to the prism a very 
yellow tinge, and was retained by it for a very long time. 
Tabasheer readily imbibes all the volatile and fat oils, and 
indeed all the fluids that I have tried. The essential oils are 
quickly absorbed, and with the exception of oil of cassia are 
as quickly evaporated, while the fat oils are slowly drawn in 
and remain a long time in its pores ; and in all these cases an 
opacity is produced by a partial absorption exactly as in the 
case of water. 
When the oils or other fluids have a colour of their own, 
or are tinged with any colouring matter, the tabasheer exhi- 
bits a similar tint, so that it is easy to communicate to it any 
colour that we please. From a solution of acetate of copper, 
it acquires the colour of the emerald ; from any of the oils 
coloured with anchusa root, it receives the tints of the ruby ; 
from oil of beech nut, the colour of the chryso-beryl ; from 
sulphuric acid, that of the pink topaz ; and from malic acid, 
that of the Brazilian topaz. These different colours may be all 
discharged by exposing the tabasheer to a red heat, and thus 
expelling the absorbed fluid to which they owe their origin. 
The opaque tabasheer, which retains its opacity when its 
pores are filled with water, acquires the most beautiful trans- 
parency from the absorption of oil of beech nut ; and it is curi- 
ous to observe a substance like chalk, and consisting appa- 
rently of a number of particles in a state of accidental 
aggregation, converted into a transparent mass, which the 
light freely penetrates in every direction. Having saturated 
a large piece of this kind of tabasheer with oil of beech nut, 
coloured with anchusa root, I laid it on a mass of lead of a 
lower temperature than that of the room. The oil instantly 
MDCCCXIX. P p 
