A. R. Hunt — Age of the Dartmoor Granites. 107 



heated water dissolved solids, superheated steam above the critical 

 temperature did not do so. The brine and freshwater inclusions 

 were therefore explicable on the hypothesis that the felspars caught 

 up compressed non-solvent steam above the critical temperature, and 

 that the quartzes caught up solvent water under that temperature. 

 Of late years, however, experimentalists seem agreed that compressed 

 gas above the critical temperature has even greater solvent properties 

 than the superheated liquid under that temperature. Thus the 

 hypothesis that the purity of the water in different minerals may be 

 owing to the inclusion of highly compressed non-solvent steam 

 above the critical point seems to break down. It seems possible 

 that certain experiments made by Mr. J. B. Hannay may clear up 

 this particular difficulty. According to Mr. Hannay, 



(1) "Gas must have a certain density before it will act as a 

 solvent, and when its volume is increased more than twice its liquid 

 volume its solvent action is almost destroyed" (Proc. Eoy. Soc. 

 vol. XXX. p. 486). 



(2) " Retaining the volume the same, the higher the temperature 

 the greater the solvent power" (Proc. Eoy. Soc. vol. xxx. p. 486). 



(3) " The vaporous state can be clearly defined as a distinct state 

 of matter." 



"A vapour over a liquid holding a coloured solid in solution is 

 colourless, but on passing the critical temperature the whole becomes 

 coloured" (Proc. Roy. Soc. vol. xxxiii. p. 321). 



Thus, according to Mr. Hannay, superheated water under the 

 critical point is a solvent liquid. Superheated steam under the 

 critical point is a non-solvent vapour. Superheated steam over 

 the critical point is a gas, and, if compressed to two volumes of the 

 liquid and less, is a solvent. 



In the case of the felspar and apatite above referred to, if these 

 minerals crystallized below the critical temperature, but under 

 pressure not sufficient to compress the vapour into liquid, they 

 would enclose compressed non-solvent vapour, which would ulti- 

 mately condense into fresh water. Another explanation of these 

 minerals enclosing plain water, even above the critical point, would 

 be the absence of chlorides, or of soluble minerals alien to the grow- 

 ing crystals, during the process of crystallization. 



Liquid inclusions of the normal type in granitoid rocks contain 

 plain water with mobile, and sometimes very active, bubbles. In- 

 clusion of salts are exceptional. The presence of fresh water during 

 crystallization of the quartz would account for the one sort ; and the 

 presence of sea water would account for the other. The difficulty 

 lies in the rapid alterations from fresh water to salt, and vice versa. 



The analogy of the marine boiler and engine, with condensation 

 of fresh water in the condenser, and possibly in the cylinders, and 

 with accumulation of brine in the boiler, suggests a possible ex- 

 planation of these complicated inclusions, viz. heat acting on salt 

 water under pressure producing rapid alternations of brine, steam, 

 and fresh water, during the crystallization of the minerals concerned. 

 It may be noticed that the variations are often so great, and the 



