hj Doubly Eefracting Crystals. S47 
fire had been continued for some time, a sort of opalescence, or 
milky opacity, was induced ; and the light, which went to the 
formation of the ordinary image, was much I'edder than that 
which formed the extraordinary ray. This effect I naturally 
ascribed to some change in the state of the carbonic acid ; 
and upon continuing the action of the heat, and watching the 
process of decomposition, I found that when the carbonic acid 
was expelled from a film about the 200th part of an inch thick, 
its surface was covered with vesicles arranged in straight lines 
^parallel to the short diagonal of the rhomboid. These vesicles 
had, in general, an elliptical form, with a cut or opening in the 
direction of their transverse axis through which the gas had 
escaped. After the spar was taken from the fire, a great num- 
ber of the vesicles burst with a noise similar to that which ac- 
companies the bursting of the indusium of the fern, and carried 
off a portion of the thin calcareous pellicle. When this pellicle 
was removed, the subjacent surface was covered with a series of 
minute parallel grooves inclined about SO'’ 57' to the short 
diagonal. In repeating this experiment, and seizing the pro- 
per time for withdrawing the spar from the fire, I never failed 
to observe the fact of the arrangement and bursting of the ve- 
sicles ; and I have no hesitation in concluding, that the carbonic 
acid is arranged in planes passing through the axis of the cry- 
stal, a result which I had formerly assumed in explaining the 
phenomena of double refraction. This method of studying the 
structure of bodies by watching them in the process of disinte- 
gration, may be found to have a very extensive application in 
chemical and mineralogical inquiries. 
The observations contained in the preceding pages, indicate 
in a manner by no means equivocal, that the colouring particles, 
of crystals, instead of being indiscriminately dispersed through- 
out their mass, have an arrangement related to the ordinary and 
extraordinary forces which they exert upon light In some spe- 
cimens the extraordinary medium is tinged with the same co- 
louring particles, and with the same number of "them as the or- 
dinary medium ; but in other specimens of the same mineral, 
• Several new facts relative to the arrangement of the colouring matter in 
crystals, will be found in the Edin. Trans, vol. ix. p. 113., now in the press. 
z 2 
