378 LETTERS ON NATURAL MAGIC. 



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 vacuities, 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. 77, gradually contracts 

 till all the vapour has been precipitated. It is curious to 



Fig. 77. 



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 expansible 

 fluid, the other denser fluid at A and C, Figs. 75, 76, 

 remains unchanged either in its form or magnitude. On 

 this account I experienced considerable difiiculty in prov- 

 ing that it was a fluid. The improbability of two fluids 

 existing in a transparent state in absolute contact, without 

 mixing in the slightest degree, or acting upon each other, 

 induced many persons to whom I showed the phenomenon 

 to consider the lines m n op, Figs. 75, 76, as a partition 



