398 
Dr. Brewster on the optical 
fragments by immersion in oil, if they do not arise from the 
existence of minute pores, which admit the oil and keep out 
the water, must be ascribed to a very considerable refrac- 
tive density in the solid parts of the substance. The expe- 
riments made with the oil at different temperatures, indeed, 
seem to prove that the refractive power of the solid parts of 
the tabasheer is equal to about 1,500, or that of oil of beech 
nut raised to the temperature which produces the maximum 
transparency ; but as the refractive power of a prism of 
tabasheer is greatly inferior to that of water, we shall now 
proceed to consider how these apparently opposite results 
can be reconciled, and what inferences can be drawn from 
them. 
When alcohol is poured into water in a glass vessel, a 
scattering of the transmitted light is immediately visible, in 
consequence of their imperfect mixture, and of the difference of 
their refractive powers ; but in a short time the union of the 
fluids is so complete, that the light is transmitted as freely as 
through either of them separately. Chemistry does not inform 
us how the particles of alcohol and of water are combined 
so as to produce this effect ; but we know that the refractive 
powerof the compound is intermediate between that of the two 
ingredients ; and hence it is certain, that the refraction of the 
incident light is produced by their joint action. If we increase 
the quantity of water successively, the particles of alcohol 
will be thrown to a greater and greater distance, and the 
refractive power of the compound will be proportionally 
diminished. Let us now suppose that all the aqueous parti- 
cles are extinguished, and that their place is occupied with a 
less refractive fluid, such as air; then it is obvious that the 
