Dr Brewster on the Phosphorescence of Minerals. 387 
the heat. Its phosphorescence, however, was entirely gme ; 
and though I placed one fragment for several days in the rays 
of the summer sun, and even exposed it to the brilliant light 
near the focus of a burning-glass, I could not succeed in obtain- 
ing from it the slightest indication of phosphorescence. When 
placed upon the heated iron, it lay quietly in its pkice without 
flying to pieces, as it never fails to do before the phosphorescence 
is extinguished. 
Although the property of imbibing light is not necessarily 
connected with that of giving it out when exposed to heat, yet 
there can be no doubt that this property is also possessed by 
several minerals. I have repeatedly observed it in the diamond 
and in blende, as others had previously done ; and we are in- 
formed by Dufay, that some emeralds imbibe light, and by 
Brugnatelli, that lapis lazuli has the same property, while 
Beccaria maintains that it is possessed by almost all substances 
whatever. 
In examining the nature of the phosphoric light emitted by 
heated minerals, by analysing it with a doubly refracting prism, 
I noticed a very singular illusion, which has already been al- 
luded to in this Number. In observing the two images of 
the luminous fragment, one of them occasionally disappear- 
ed during the revolution of the prism, but according to no 
regular law. I was therefore led to believe that the fluor-spar 
emitted, by fits, light polarised in different planes. A result so 
extraordinary required every kind of verification, and I soon 
found that no such property was indicated when the light was 
analysed either by reflection or by a prism of calcareous-spar, 
in which one of the pencils had been extinguished The dis- 
appearance of one of the images had therefore arisen from its 
having been seen indirectly, (as one of the two must always be,) 
in consequence of which the retina occasionally lost the power 
of perceiving it-f* *. 
The following are among the principal results of the preced- 
ing experiments. 
* See Philosophical Transactions 1819, p. 14'8. 
*j- See page 325, of this Number. ' 
