VAPOUR CAVITIES IN MINERALS. 377 



in common fluids, and A m n, Cop, 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. 76. The 

 fluids are shaded, as in these two figures, when they are 

 seen by light reflected from their surfaces. 



Fig. 75. 



When the cavities are large, as in Fig. 77, compared 

 with the quantity of expansible fluid m n p o, the heat 

 converts the fluid into vapour, an effect which is shown by 

 the circular cavity V becoming larger and larger till it 

 fills the whole space m no p. 



Fig. 76. 



When any of these cavities, whether they are filled with 

 fluid or with vapour, is allowed to cool, the vacuity V 

 reappears at a certain temperature. In the fluid cavities 

 the fluid contracts, and the small vacuity appears, which 

 grows larger and larger till it resumes its original size. 

 When the cavities are large several small vacuities make 



2 c 



