174 Transactions of the Royal Microscopical Society. 



critical point, the liquid has the appearance seen in Fig. 5, hut the 

 boiling is shown in Fig. 6 — the spherules called gas and liquid are 

 passing in the direction of the arrows nearest them. The drawing, 

 Fig. 7, represents a cavity seen in one of my specimens of quartz ; 

 the contents are undergoing the apparent boiling. The conditions 

 favouring this singular mode of condensation seem to be, first, that 

 the greater part of the carbonic acid shall be in the liquefied state 

 at ordinary temperatures so that the liquid expands greatly on 

 approaching the critical point ; second, that the cooling shall be 

 sudden. 



Cavities containing liquefied carbonic acid may be divided into 

 two classes, wet and dry cavities, according to the absence or presence 

 of water. The appearance of the liquid in a dry cavity differs much 

 from that in a wet one. Thus in a dry cavity the liquid presents 

 a convexly curved surface to the gas, in a moist one a concave 

 surface. While the carbonic acid in sapphires and rubies seems 

 generally to be dry, that met with in quartz and other minerals is 

 more frequently wet. 



Another means of ascertaining the critical temperature of the 

 liquid in fluid cavities was resorted to. It consisted in making a 

 water-tight cell with glass sides, which would contain besides the 

 mineral under examination the bulb of a small thermometer three 

 inches in length and graduated into one-tenths of a degree Centi- 

 grade, between 29° and 35" C. An inlet and outlet tube of india- 

 rubber conveyed a stream of warm water, forced through the cell 

 from a small flask by means of the pressure of a large india-rubber 

 finger-pump or syringe. The entrance and exit for air to and from 

 the syringe was by valves in different branches of a "f tube. 



The walls of the cell were made by boring a hole an inch in 

 diameter through an india-rubber cork of the diameter of 1|- inch. 

 Two perforations one-eighth of an inch in diameter were made in 

 the side of this to admit the water tubes, and a third for fixing the 

 thermometer in. The glass slides placed top and bottom of the 

 ring were firmly fixed by passing stout india-rubber bands over 

 them. When the cell was placed on the microscope stage, a power- 

 ful Coddington lens was so arranged in position near the thermo- 

 meter that without the slightest movement the cavities could be 

 watched through the microscope with the left eye, and simulta- 

 neously the mercury in the thermometer with the right. The 

 cavity under examination was so arranged that it appeared just 

 upon the edge of the lens. Admirable though this arrangement 

 seems, it does not answer quite so well as one might expect. The 

 volume of water in the cell is so small, that it changes temperature 

 more readily than the slowly conducting mineral. 



My original plan of immersing the mineral in a considerable 

 volume of water at temperatures just above and below the critical 



