VAPOUR CAVITIES IN MINERALS. 



333 



temperatures there is always a vacuity V, like an 

 air-bubble in common fluids, and A m n, C o p, 

 the second fluid occupying the angles A and C. 

 When heat such as that of the hand is applied to 

 the specimen, the vacuity V gradually contracts 

 in size, and wholly vanishes at a temperature of 

 about 80, as shown in Fig. 81. The fluids are 

 shaded, as in these two figures, when they are 

 seen by light reflected from their surfaces. 

 Fig. 81. 



When the cavities are large, as in Fig. 82, 

 compared with the quantity of expansible fluid 

 m n p o, the heat converts the fluid into vapour, 

 Fig. 82. 



n p 



an effect which is shown by the circular cavity V 



