142 JOURNAL OF THE [June, 



i,oou th of an inch, often 50,000 th of an inch in diameter, is the so- 

 called spontaneous motion with which the inclosed bubbles are 

 affected. Some may be seen under a high power (a ith-inch or 

 xVth-inch objective) vibrating gently, others rolling to and fro, 

 and others dashing from side to side in restless motion. This 

 motion may be very rarely seen even in cavities visible under 

 a Ids-inch objective, or a magnifying power of about sixty-five 

 diameters. Its cause has been connected pretty certainly with 

 unequal thermal conditions on the sides of the cavities, produc- 

 ing alternations of evaporation and condensation within the space 

 of the bubbles. Their motion thus serves as an index of the 

 delicate balance which must prevail throughout the fluid, under 

 pressure of its own vapor, within the cavity. Occasionally, 

 bubbles in motion have been seen to stop, even permanently ; 

 others have begun their motion under the eyes of the observer, 

 and continued it as long as they remained under examination. 

 I am not aware that this spontaneous motion of the bubbles has 

 yet been observed in the fluid-cavities of artificial crystals. 



A curious phenomenon has been occasionally noticed by the 

 experimenter while heating the fluids in a cavity of rock-crystal. 

 At a certain temperature, in one case as high as 150° C, the gas- 

 bubble becomes more dense than the liquid and sinks to the 

 bottom of the cavity. " The cause of this motion appears to be 

 that the bubbles consist of a gas so highly compressed that it is 

 nearly of the same density as water at the ordinary temperature. 

 On heating, the water expands, thus still further condensing the 

 gas in the cavity, which then becomes heavier than the liquid, 

 and consequently sinks in it.'"* 



In closing, I may refer briefly to a few practical applications 

 of the facts which have been discovered in reference to fluid- 

 cavities. As in the sedimentary rocks fossils are necessarily 

 relied upon for the determination of their life-history, so espe- 

 cially in the crystalline rocks have these fluid-cavities served a 

 most useful purpose for determining the genetic history — the 

 conditions which obtained during their formation or alteration. 

 So exact is that statement that geologists have ascertained, 

 within perhaps one thousand feet, the exact depth at which 

 certain rocks were formed in England, and the exact pressure to 

 which others in Scotland were subjected (represented in the 



"W. N. Hartley : Jour. Chem. Soc, London, 1877, pp. 837-50. 



