$9° Dr. Brewster on the optical 
retired from the surface of the tabasheer into its interior, and 
the ; transparent mass became opaque like apiece of red brick. 
Upon removing it from the lead into its former temperature, 
the oil returned to the surface, and the tabasheer 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 sufficiently 
high degree of heat. The phenomena which have now been 
described admit of a satisfactory explanation from the diffe- 
rence between the expansion of the oil and that of the taba- 
sheer ; but the effect appears to be too great to arise from 
this cause, and I am rather disposed to ascribe it to a varia- 
tion in the capacity of the tabasheer for the oil by a change 
of temperature. 
In order to observe the nature of the penumbral boundary 
which might be supposed to separate the opaque from the 
transparent part, 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 became diaphanous, and the transparency 
gradually pervaded the opaque mass. As soon as the opa- 
city disappeared, I allowed one extremity of it to cool ; the 
transparency immediately disappeared 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 
