294 
Dr. Brewster on the optical 
I was now anxious to see the effect produced upon taba- 
sheer by the absorption of iodine gas, and for this purpose I 
took several opaque and transparent fragments, and having 
saturated some cf them with moisture, and left others dry.. 
I placed them in different glass tubes, which were hermeti- 
cally sealed after a portion of iodine had been introduced. 
Previous to the application of heat the tabasheer assumed a 
yellow tinge, which deepened into a pale orange, and a veined 
structure appeared in one of the fragments. When the 
iodine was converted into gas by heat, the colour of the 
fragments grew more and more red ; the transparent pieces 
were like garnets, and the opaque ones like fragments of 
red brick ; and after standing two or three days the opaque 
pieces became perfectly transparent. The iodine vapour, 
therefore, seems to have taken the place of the water in the 
wetted fragments, and of the common air in the dry ones ; 
and it appears to have retained its gaseous form within the 
tabasheer, when the external gas had returned to the solid 
state. Upon taking the tabasheer out of the glass tubes, the 
iodine was slowly expelled, a yellow tinge appeared even 
after 30 hours exposure to the air, and it was not entirely 
removed by immersing the fragments in water. 
The difference in the properties of the opaque and the 
transparent tabasheer, rendered it desirable to have accurate 
measures of their specific gravities, and I have been fortunate 
enough to procure them to a great degree of exactness, 
through the kindness of my friend, Mr. James Jardine, who 
obtained for me the following results. 
