334 LETTERS ON NATURAL MAGIC. 



becoming larger and larger till it fills the whole 

 space m n o p. 



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 tem- 

 perature. 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 vacui- 

 ties make their appearance and gradually unite 

 into one,' though they sometimes remain separate. 

 In deep cavities a very remarkable phenomenon 

 accompanies the reappearance of the vacuity. 

 At the instant that the fluid has acquired the 

 temperature at which it quits the sides of the 

 cavity, an effervescence or rapid ebullition takes 

 place, and the transparent cavity is for a moment 

 opaque, with an infinite number of minute vacui- 

 ties, which instantly unite into one that goes on 

 enlarging as the temperature diminishes. In 

 the vapour cavities the vapour is reconverted by 

 the cold into fluid, and the vacuity V, Fig. 82, 

 gradually contracts till all the vapour has been 

 precipitated. It is curious to observe, when a 

 great number of cavities are seen at once in the 

 field of the microscope, that the vacuities all dis- 

 appear and reappear at the same instant. 



While all these changes are going on in the 

 expansive fluid, the other denser fluid at A and 

 C, Fig. 80, 81, remains unchanged either in its 

 form or magnitude. On this account I expe- 

 rienced considerable difficulty in proving that it 

 was a fluid. The improbability of two fluids 

 existing in a transparent state in absolute con- 

 tact, without mixing in. the slightest degree, or 



