by doubly refracting crystals. 
27 
have never failed to observe the fact of the arrangement 
and bursting of the vesicles, and I have no hesitation in con- 
cluding, that the carbonic acid is arranged in planes passing 
through the axis of the crystal, a result which I had for- 
merly assumed in explaining the phenomena of double re- 
fraction. This method of studying the structure of bodies by 
watching them in the process of disintegration, may be found 
to have a very extensive application in chemical and mine- 
ralogical enquiries. 
The observations contained in the preceding pages, indicate 
in a manner by no means equivocal, that the colouring par- 
ticles of crystals, instead of being indiscriminately dispersed 
throughout their mass, have an arrangement related to the 
ordinary and extraordinary forces which they exert upon light. 
In some specimens the extraordinary medium is tinged with 
the same colouring particles, and with the same number of 
them as the ordinary medium : but in other specimens of the 
same mineral, the extraordinary medium is either tinged with 
a different number of particles of the same colour, or with a 
colouring matter entirely different from that of the ordinary 
medium. In certain specimens of topaz, the colouring matter 
of the one medium is more easily discharged than that of the 
other ; and in two specimens of emerald , the colouring matter 
which tinges the ordinary medium in the one, tinges the ex- 
traordinary medium in the other, and vice versa. 
All those crystals in which the colouring matter of the one 
medium differs either in character or intensity, possess the 
property of absorbing the two tints according to the laws 
