and physical properties of tabasheer. 2 gg 
refractive power of the compound will be inferior to that of 
water, and will approach to that of air, in proportion to the 
quantity of air united with the alcohol. 
This hypothetical combination of air and alcohol, repre- 
sents exactly what I conceive to be the condition of the dry 
and transparent tabasheer. The refractive power of the 
solid parts is not far from 1,500 ; but the substance is so 
formed, that its particles are kept at a great distance by 
means of vacuities filled with air, and arranged in such a 
manner, that the light passes from the particles of tabasheer 
into the particles of air without suffering refraction, in the 
same manner as it passes from the particles of alcohol to the 
particles of water, when these fluids are chemically united. 
That there is not a chemical union between the air and the 
tabasheer, is certain, because the air may be displaced, and 
the transparency preserved in vacuo ; and hence we may 
draw r the important inference, of which we have no other 
example in physics, that the tabasheer and its included air 
exercise a joint action upon light, in the same manner as if 
they were in a state of chemical union. I have the honour 
to be, &c. &c. &c. 
DAVID BREWSTER. 
Edinburgh, March z, 1819. 
