Carbonic Acid in Mineral Cavities. By Walter N. Hartley. 173 



elusion wliicli lie and Mr. Sorby arrived at was a just one. By a 

 very simple contrivance I have been enabled to detect the presence 

 of liquid carbonic acid in many very small cavities containing water. 

 This consists of a glass tube about three-eighths of an inch in dia- 

 meter and twelve inches long ; it is drawn out to a jet at one end of 

 about one-sixteenth of an inch aperture, the jet being bent at an 

 obtuse angle. To prevent the glass being softened and bending 

 when heated, it is covered for four inches in its central part by a 

 piece of brass tube which slides on not too easily. The straight 

 end of the tube is somewhat pointed, and passes through an india- 

 rubber cork fitting into a universal joint upon a stand having a 

 sliding motion in the upright so that it may be raised or lowered 

 at will. This end of the glass tube which has passed through the 

 cork has a piece of india-rubber tube slipped over it fifteen inches 

 long, and to this is attached a ball syringe whereby air may be 

 drawn in and discharged again. By heating the metal tube with 

 a spirit lamp or Bunsen burner, the air discharged will be heated 

 and may be directed on to the object while undergoing examination 

 beneath the microscope without any displacement whatever, by 

 which means a high power may be used for the examination of 

 small cavities. By noticing the number of ballfuls of air necessary 

 to vaporize a known specimen of carbonic acid, one may, if these be 

 sufficient to vaporize the liquid in small cavities, be certain that 

 the temperature is not greatly difiierent. It is easy to demonstrate 

 the presence of small quantities of carbonic acid mixed with water 

 in cavities no larger that ttV o of an inch in their greatest dia- 

 meter. 



After carbonic acid has passed its critical temperature, if it be 

 cooled suddenly it condenses with a motion resembling ebullition. 

 This is best seen in deep cavities. Messrs. Sorby and Butler have 

 observed this phenomenon.* Having attentively studied it in dif- 

 ferent cavities, I have come to conclusions as to the meaning of it, 

 "When the gas is chilled, a sort of mist forms throughout the space ; 

 the individual spherules of this mist grow so large that they begin 

 to touch each other, to coalesce, and to gravitate. They of course 

 at the same time entangle gas, and as they descend to the lower 

 part of the cavity the spherules of gas (bubbles) take an opposite 

 direction ; consequently when a portion of the liquid has collected 

 at the lower end and gas at the upper, there are showers of liquid 

 descending into and streams of bubbles rising out of the liquid. 

 In two or three seconds the movements have ceased. In Figs. 5 

 and 6 are given representations of a fluid cavity in topaz belong- 

 ing to Mr. James Bryson, of Edinburgh, to whom I am much 

 indebted for allowing me to examine some of his valuable speci- 

 mens. When at a temperature two or three degrees below the 



* ' Monthly Microscopical Journal,' vol. i., p. 222. 



