ON LIQUID CARBONIC ACID IN ROCKS AND MINERALS. 233 



Critical point. 



Topaz 28° C. 



Topaz 28° C. and 2G°-5 



Topaz 27°-55 



Tourmaline 27°-27 



Tourmaline 26°-9 



Sapphire between 30°-5 and 31° 



Sapphire between 25 0, 5 and 26° 



Sapphire 29°-5 



Rock crystal 30°-95 



Rock crystal 30°-95 



Rock crystal 32°-5 



Rock crystal 33°-7 



Rock crystal 29° 



Rock crystal 30°-95 



Beryl 30°-92 



Rock crystal from India 30°-0 C. 



Topaz, Aberdeen 29°-l 



Oriental White Topaz 28 c -2 



Rock crystal 21°-0 



It seemed to be very desirable to ascertain whether the presence of liquid 

 carbonic acid in rocks was not of frequent occurrence, whether, in fact, the 

 immense number of minute cavities dispersed through quartzite, granites, 

 and porphyries, which are usually cousidered as containing water, may not 

 often contain liquid carbonic acid, or whether the occurrence of liquid car- 

 bonic acid in rocks might not be characteristic of certain formations. 



Method of Working, Sfc. — The microscopic observations of Bryson on the 

 quartz porphyry of Arran, also of Zirkel on Labradorito, tend to show that 

 if some means could be devised of readily recognizing minute quantities of 

 this substance it would be frequently met with. The apparatus shown at 

 the last meeting of the British Association was made use of. Its action 

 raises the temperature of the specimen under examination to above the 

 critical point of carbonic acid, and but for a single instant of time only if 

 desirable. So marked is the change in appearance of cavities containing 

 liquid carbonic acid when a current of warm air is blown upon them, that a 

 layer of carbonic acid no larger than ^^ of an inch in diameter may bo 

 detected. 



It has been necessary to examine a great variety of rocks, and very thin 

 cctions have been cut from about two hundred different specimens during 

 the past year. These were polished but not covered with a thin glass, be- 

 cause a better examination may be made with high microscopic powers. A 

 ^-inch object-glass was made by Messrs. R. and J. Beck, after the pattern of 

 one made according to Mr. Sorby's directions. Its definition is perfect at 

 any depth in a reasonably well-cut rock-section. A considerable number of 

 minerals were examined, including about 30 sapphires, a like number of 

 zircons, 60 garnets from the Cape of Good Hope, several topazes and sec- 

 tions of fluor-spar, sulphate of baryta, and arksutite (a fluoride of aluminium, 

 calcium, and sodium) from Greenland. 



Motion of bubbles in fluid-cavities under the influence of heat. — Incidentally 

 this inquiry has led to the discovery of curious facts concerning the motion 

 of the bubbles in fluid-cavities when influenced by a source of heat. An 

 extensive scries of experiments were made, the details of which are fully 



