ON LIQUID CARBONIC ACID IN ROCKS AND MINERALS. 235 



tides, and these vibratory motions afford an ocular demonstration of the 

 continual passago of heat through solid substances. 



A further continuation of this research was extended to the conditions 

 under which minute solid particles exhibit the Brownian movement. It 

 was found that solid particles are subject to the same influences and behave 

 in the same way as minute bubbles, a fact which was anticipated. As to the 

 cause of tho movement there can be no doubt, since the very recent investi- 

 gations of M. Delsaulx of Louvain, on the thermodynamic origin of the 

 Brownian movement, lead to the samo conclusion ; but with regard to the 

 modus operandi of this cause it will be well to reserve further statements 

 until an cxbaustive study of JM. Delsaulx's views may warrant a decision. 



General views concerning the. occurrence of liquid carbonic acid in minerals. 

 — Liquid carbonic acid is not of common occurrence in rocks and minerals, 

 although occasionally met with. 



The critical point is rarely to be found exactly the same as that deter- 

 mined by Professor Andrews, and it ranges from 32° 0. in a sapphire to 

 21° C. in quartz. 



The conditions of pressure under which the liquid carbonic acid exists are 

 very varied : thus, in some cases the quantity of liquid in proportion to gas 

 is so small that a rise of 5° or 6° C. above 16° causes it to disappear by eva- 

 poration. In other cases it may be made to expand and fill the cavity at or 

 about its critical point ; and in one instance, in the case of a piece of felstone 

 from Snowdon, it was found that tho liquid had expanded to the fullest 

 extent possible at so low a temperature as 18° C. 



Continuity of the gaseous and liquid states of matter exemplified in certain 

 specimens. — In other instances noticed in large cavities in a white topaz, the 

 liquid was in sufficient quantity to fill the cavity at 2° or 3° C. below its 

 critical point. 



Under such circumstances when the liquid was completely converted into 

 gas it condensed on cooling ivitJiout undergoing any visible change. 



It may well be asked how the fact of this change of state was ascertained. 

 The following description of experiments will explain all. 



In a section of a colourless oriental topaz containing a large number of 

 cavities, one of large size was easily studied with a magnifying-power of 40 

 diameters. A jet of warm air raised the liquid above its critical point. 

 After waiting for a minute, during which no change of any kind was seen in 

 the cavity, a very slight puff of warm air was directed on to the specimen, 

 and immediately a crowd of little bubbles made their appearance in its centre ; 

 these instantly vanished, closed up in fact, but could be reproduced again 

 and allowed to disappear as rapidly and as often as one could desire. The 

 jet of air coidd be regulated so gently that only two or three bubbles were 

 formed. It is evident, then, that the cavity is completely filled with liquid. 

 When the jet of warm air was forcible no change was seen to take place, but 

 a gentle warmth no longer caused the formation of bubbles therefore the 

 cavity must have been filled with gas. It is evident, then, that in the first 

 case the gas had passed into the liquid state without breach of continuity, 

 and in the second the passage, in a reverse direction, from liquid to gas had 

 taken place in like manner. 



Thus one sees beautiful illustrations in natural specimens of Professor 



Andrews's famous law of continuity in the gaseous and liquid states of matter. 



On the temperature of formation of rocks and minerals. — Regarding the 



proportions of gaseous and liquid carbonic acid, an important generalization 



has been arrived at. 



