] 72 Transactions of the Royal Mieroscojncal Society. 



tion and contraction of the pupil of the eye. Since the connection 

 between the cavities has been made by excessive heating, the 

 expansion and contraction cannot be shown ; the hquid at once 

 begins to vaporize, when warmed, and even boils, as is shown in 

 Fig. 4. The following observation of Thilorier explains this. 

 When a tube containing liquid carbonic acid is one-third full, at 

 0'^ C, it constitutes a retrograde thermometer, in which increase of 

 temperature is shown by diminished volume, consequent on the 

 vaporization of the hquid, and vice versa; while if the tube be 

 two-thirds full, a normal thermometer of great sensitiveness is the 

 result, the liquid expanding by heat in this case.* 



Very careful observation several times repeated has shown that 

 on the approach of a warm substance, causing the liquid in the 

 larger cavity to be vaporized gradually, the curvature of the surface 

 in contact with the gas becomes reduced very much, and at the 

 same time rendered less plainly visible, as shown by h, Fig. 3. 



There was also noticed a faint flickering shadow in the point 

 of the cavity, when the liquid was about to condense. Professor 

 Andrews has noticed such effects during the vaporization and con- 

 densation of liquid carbonic acid. In the same section of quartz 

 there were observed upwards of fourteen smaller cavities, containing 

 liquid carbonic acid, together with water in different proportions. 

 There are two such cavities shown in Fig. 1 ; in each case the space 

 marked a contains carbonic acid as gas ; h, the same substance 

 liquefied and floating on the water, which, being indicated by c, is 

 seen to be occupjdng the remaining space. 



According to Thilorier, the specific gravity of liquid carbonic 

 acid is ' 83 at 0^ C, and • 6 at 30^ C, water being taken as 

 unity. The constant position of this liquid in the cells being 

 uppermost is in accordance with this. 



Volatile fluids have been noticed in mineral cavities by Sir 

 David Brewster,t by the late Mr. Alexander Bryson,J and by 

 Messrs, Sorby and Butler, who came to the conclusion that the 

 liquid in a particular cavity in a sapphire was really liquid carbonic 

 acid, because it possessed a remarkable rate of expansion between 

 0° and 30^ C, Thilorier has shown that the expansion of liquid 

 carbon dioxide between 0^ and 30" C, is such, that 100 volumes 

 become 145. Sorby found that 100 volumes of the liquid he ex- 

 amined became 150 at 30° C, 174 at 31° C, and 217 at 32° C.§ 



Through the kindness of ]\Ir. Butler, I have had the advantage 

 of examining some of the best specimens from his unique collection 

 of stones with fluid cavities, and I have no doubt that the con- 



* ' Ann. Chim. Phys.' [2], Ix. 249. 



t 'Trans. Eoyal Society of Edinburgh,' vol. x., p. 1, 1823. 

 X 'Proceedings of the Royal Society of Edinburgh,' 1860-1. 

 § ' Proc. Royal Society,' vol. xvii., p. 299 ; also ' Transactions of the Royal 

 Microscopical Society,' vol. i., p. 222. 



