63 
replaced the material of glass-cavities, as suggested by Vogelsang*. Ln the 
specimens which we have examined, each of the cavities contains what is 
no doubt an aqueous saline solution, and, as shown by fig. 8, one or more 
cubic crystals, probably chloride of potassium, which dissolve on the 
application of heat, and are deposited again on cooling. These cavities 
are thus analogous to those met with in the quartz of some granite, and in 
the minerals of blocks ejected from Vesuvius ; and it seems difficult, if not 
impossible, to explain them except by supposing that a strong saline solu- 
tion was caught up by the mineral at the time of its formation. In some 
cases the amount of such saline matter is so great in comparison to the 
liquid, that a high temperature would be requisite to make it all dissolve. 
It also seems probable that, if water could penetrate into such crystals, it 
would soon be lost when they were kept dry. This certainly occurs in 
some soluble salts, especially those containing combined water, and in some 
minerals of loose texture ; but we have never seen evidence of it when 
fluid- cavities are completely inclosed in hard and dense substances like 
quartz or emerald. Though in some instances the size of the bubbles does 
not bear a uniform relation to that of the cavities, yet in many cases the 
general proportion is very similar in each specimen ; and the exceptions can 
easily be explained by supposing that occasionally small bubbles of gas 
were caught up along with the water, or that there was some variation in 
either temperature or pressure during the growth of the crystal ; all of 
which conditions were discussed in Mr. Sorby's paper already referred to. 
We have not had the opportunity of studying many examples of cavities 
which contain two fluids, probably water and liquid carbonic acid, and 
therefore forbear to say much about them. According to Brewster f the 
temperature at which those in topaz become full, corresponds very closely 
withwhat we have observed in the case of sapphire, so that the carbonic 
acid might have been inclosed either as a highly dilated liquid, or as a 
highly compressed gas ; but since the other liquid has deposited crystals 
which dissolve on the application of heat +, it seems most probable that 
the water was caught up in a liquid state, sometimes perhaps holding a 
considerable amount of carbonic acid in solution as a gas. 
On the whole therefore, the various facts described in this paper seem to 
* Philosophic der Geologie und mikroskopische Gesteinsstudien, (Bonn* 
1867) pp. 155, 196. 
+ Trans. Hoy. Soc. Edin. vol. x. p. 1 et seq. 
X See Brewster's paper, Phil. Mag. 1847, vol. xxxi. p. 497- 
