394 A. It. Hunt — The Crystallisation of Granite. 



as carbonic acid should do. I am indebted to Mr. C. W. Priestley, 

 F.C.S., for information as to the latest determinations of the critical 

 temperature of C O2 and of its pressure at the critical temperature. 

 The critical temperature is 87-75° F., and the pressure 77 atmospheres 

 or 1,155 pounds to the square inch. In the case of the larger cavitiea 

 Professor Hartley describes the strain on the including crystal as 

 visible in the polariscope. The first experiment, viz. that as to the 

 disappearance, is readily made. We take an inclusion with water 

 and C O2, and mark the position of the slide on the microscope 

 stage so that we can easily replace it. We warm the slide over 

 the microscope lamp and replace it. The double liquids are gone. 

 The carbonic acid, as gas, is apparently dissolved in the water ; 

 but, while we watch, the carbonic acid often returns with a jump,. 

 The suddenness with which the carbonic acid often reappears shows 

 that the passage from above to below the critical temperature is 

 very closely defined. It is not a question of the absorption by the 

 water of a little more or less gas. We find further that chemists 

 have experimented largely with various liquids and gases as to 

 critical temperatures, so that the general principle of critical 

 temperatures is well understood. 



So far as I can follow the observations of Messrs. Sorby and Hartley 

 on carbonic acid, I find them correct ; so I am favourably disposed 

 towards their interpretation of the behaviour of water, and to believe 

 that a liquid alone is competent to dissolve and deposit free salts. 

 This view is indirectly confirmed by the fact that volcanoes dis- 

 charge hydrochloric acid and chlorine, substances which Dana 

 attributes to the dissociation of marine chlorides which have obtained 

 access to the lava column. If, however, the chlorides of sodium and 

 potassium are dissociated more or less into their elements at volcanic 

 temperatures, say, into hydrogen, chlorine, and soda or potash, and 

 if these substances are then individually dispersed throughout the 

 liquid lava, it would require a miracle for them all to come together 

 again, to be dissolved by water as chlorides, and then deposited as 

 solid crystals in the various minerals which occasionally contain 

 them. It would require a miracle even were the substances retained 

 in the lava, but, according to Dana, volcanoes get rid of both the 

 gases concerned, both as hydrochloric acid and separately as hydrogen 

 and as chlorine. 



If, however, anyone can prove the existence of chlorides which have 

 been deposited above the critical temperature of water, we, with all 

 adverse theories, must bow to the fact and accept the miracle. But, 

 when we examine General McMahon's facts, we find them all in favour 

 of the orthodox doctrine. Upon the beryl, containing gas, and fluid 

 inclusions with deposited crystals (species not mentioned), we find 

 crystallised the minerals quartz and muscovite, both of which 

 minerals occur in schists and other non-igneous rocks. Although 

 the dry-fusion temperatures of quartz and muscovite are high, their 

 wet crystallising temperatures may be quite low. Indeed, they 

 must sometimes be so or they would not occur where they do. 

 Fouque and Levy mention the crystallisation of quartz at 



