236 report — 1877. 



In rock-crystal, in arksutite, in felstone from Snowdon, and in some 

 topazes and beryls the carbonic acid is not contained in every cavity, though 

 ■water is seen in them all. In one topaz it was noticed that nearly all the 

 cavities contained merely a trace of water, but there was a sufficiency of 

 liquid carbonic acid to occupy two thirds of their capacity at 16° C. One or 

 two cavities, however, of large size were noticed which contained one third 

 water, one third gaseous and one third liquid carbonic acid. I believe, for 

 reasons I am about to state, that all these substances were formed by the 

 action of a temperature below 340° C. In sapphires, in tourmalines, and in 

 some other topazes the condition of things is different. Irregular though 

 the cavities may be, it is easy to see that they have about the same propor- 

 tion of gas, of liquid carbonic acid, and of water, and minute search shows 

 that there is not a single cavity which does not contain in some proportion 

 all of these substances. In a colourless and clear topaz there were discovered 

 thousands of perfectly cylindrical tube-like cavities, round at each end. 

 In the case of fifty-two cavities, as far as lineal measurement could decide, 

 they each contained the same proportions of carbonic acid liquid, carbonic 

 acid gas, and water. Hence at the time they were enclosed in the mine- 

 ral these fluids must have existed in the state of a homogeneous vapour. 

 This of necessity places the temperature of formation of the mineral some- 

 where above 342° C, the critical point of water. In other cases in which 

 the cavities differ in the nature of their contents, the water at the time of 

 the formation of the mineral must have been in the liquid state. It is pos- 

 sible to determine within certain limits the temperature which a rock or 

 mineral has endured (and that, too, very easily) if liquid carbonic acid is 

 found enclosed in it. 



