6 DR BREWSTER on the Existence of Two New Fluids 



\r 4 ffi. n '4- Temperature at which the 



Nature of the Cavities. 



Vacuity reappeared. 



8. Colourless Topaz from Brazil, with only one 

 cavity j^th of an inch long, 5 ] g d of an inch 

 broad, and ? \th of an inch wide, 79 



4. Topaz from New Holland, with large and 



rugged cavities, 79 



5. Topaz from New Holland, with a very flat 



cavity, 81 | 

 A very long and very irregular cavity in the 



same crystal, 82 i 



6. Another colourless Topaz from Brazil, 83 j 



7. Another colourless Topaz from Brazil with a deep cavity, 83 f 



The reappearance of the vacuity at different temperatures in dif- 

 ferent cavities of the same crystal, admits of an easy explana- 

 tion. In those which are of the same size and form, and equi- 

 distant from the cooling surface, the vacuities disappear at the 

 same time ; but in those which are deep, and in those which, 

 though shallow, are near the cooling surface, the vacuities reap- 

 pear at a lower temperature. In very shallow cavities, the ad- 

 hesion of the fluid to the sides of the cavity prevents the va- 

 cuity from reappearing so soon as it would otherwise do ; while 

 in cavities that have a rough or irregular bottom, they reappear 

 earlier. 



When the cavities are very small and narrow, only one va- 

 cuity reappears ; but when they are large, several small circular 

 vacuities make their appearance, and gradually unite into one, 

 though sometimes they remain permanently separate. When 

 the cavities are deep, a very remarkable phenomenon accompa- 

 nies the reappearance of the vacuity. At the instant that the 

 fluid has acquired the temperature at which it quits the sides of 

 the cavity, a rapid ebullition takes place, and the transparent ca- 

 vity is for a moment opaque, with an infinite number of minute 



