98 Dr Brewster on the General Properties of Tabasheef. 
opacity is produced by a partial absorption exactly as in tb© 
case of water. 
When the imbibed fluids have a colour of their own, or are 
tinged with any colouring matter, the tabasheer exhibits a simi- 
lar tint, so that it is easy to communicate to it any colour. 
From a solution of acetate of copper, it acquires the colour of 
the emerald ; from oils coloured with anchusa root, it receives 
the tints of the ruby ; from oil of beech-nut, the colour of the 
chrysoberyl ; 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 cu- 
rious to observe a substance like chalk, and consisting appa- 
rently of a number of particles in a state of accidental aggre- 
gation, converted into a transparent mass, which the hght 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 retired from the sur- 
face of the tabasheer into its interior, and the transparent mass 
became opaque like a piece of red brick. Upon removing it into its 
former temperature, the oil returned to the surface, and the ta- 
basheer resumed its transparency. If, on the other hand, we 
place it in a higher temperature than that of the room, a part 
of the oil will be discharged, and when it is brought back to its 
first temperature, it will become opaque like a piece of brick. 
Even when a small part of the oil remains, the transparency 
may be readily restored by the application of a sufficient de- 
gree of heat. The phenomena which have now been described, 
admit of a satisfactory explanation from the difference between 
the expansion of the oil and that of the tabasheer ; but the ef- 
fect appears to be too great to arise from this cause, and I am 
rather disposed to ascribe it to a variation in the capacity of the 
tabasheer for the oil by a change of temperature. 
