96 Dr Brewster on the Existence of Two New Fluids 
magnitude ; and it deserves particular notice, that the evanes - 1 
cence and reappearance of the vacuity takes place simultaneous- 
ly in many hundred cavities, of the same general form, which 
may be seen in the field of view. 
In order to obtain an accurate measure of the temperature at 
which the vacuity reappears, which is almost the same as that at 
which it vanishes, our author plunged the topaz in heated water, 
and, by means of an accurate thermometer, obtained the follow- 
ing results : 
Temperature at which the 
Nature of the Cavities. Vacuity reappeared » 
t . Topaz from New Holland, With shallow cavities, - 74£o 
2. Blue Topaz from Aberdeenshire, with cavities of different forms, 74 Q — 82° 
3. Colourless Topaz from Brazil, - - 79g° 
4. Topaz from New Holland, with large and rugged cavities, 79|° 
5. Topaz from New Holland, with a very flat cavity, - 81|;o 
6. Another colourless Topaz from Brazil, with a deep cavity, 83°| 
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 accom- 
panies 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 
cavity is for a moment opaque, with an infinite number of mi- 
nute vacuities, which instantly unite into one vacuity, that gra- 
dually enlarges as the temperature diminishes. 
In order to determine the expansion which takes place by a 
given increment of temperature, our author measured the rela- 
tive size of the vacuity, and the cavity at the temperatures of 
50° and 80°, the temperature at which the fluid had expanded 
so as wholly to fill the cavity. In many cases this could be es- 
timated with tolerable accuracy, and it may be stated in gene- 
ral, from the estimates and measures taken by different persons, 
to whom the cavities were shewn, that the fluid expands fully 
One fourth of its size, by an increment of 80° of heat ; and that 
it is nearly 32 times more expansible than water , by an incre- 
ment of 3 0° of heat at the temperature of 50°, 
