216. Fluids in the Cavities of Minerals. 
diminishes, and being in a short time reduced to a physical 
point, it entirely disappears. When the fluid again cools, by 
withdrawing the hand, it of course contracts and quits the 
sides of the cavity. The vacuity V re-appears, increasing 
till it resumes its former magnitude ; and it deserves prvige 
lar notice, that the evanescence and re-appearance of the 
cuity takes place simultaneously in many hundred cavities, vo 
the same general form, which may be seen in the field of 
view. 
Tn order to obtain an accurate measure of the temperature 
at which the vacuity re-appears, which is almost the same as 
that at which it apg our author plunged the topaz in 
_ heated water, and, b ans of an accurate thermometer, 
obtained the following sekaiag 
Temperature at which the 
ure of the cavities, vacuily re-appeare 
1. Topaz ee New-Holland, with shallow ees 743° 
2. Blue ee from Aberdeenshire, with ca 
di 
ifferent forms, 74°-82° 
3 Coliried topaz from Braz 793° 
4. Topaz from ‘Kew-Holland, pore large and rug- 
ged cav 795° 
5. Topaz from New-Holland, with a very flat cavity, 814° 
6. colourless topaz from Brazil, with a dee , 
Cavity, 837° 
the cavities are very small and narrow, only one 
vacuity re-appears ; 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 
accompanies the re-appearance of the vacuity. At the in- 
stant that the fluid has acquired the temperature at which it 
quits the sides of the cavity, a rapid ebullition a ie place, 
: transparent cavity is for a moment opaque, with an 
infinite number of minute vacuities, which instantly unite in- 
to one vacuity, that gradually enlarges as the temperature di- 
minishes, 
_ bee to determine the expansion which takes place by 
increment of temperature, our author measured the 
ge eo: of the eee and the cavity at the tempera- 
temperature at which the fluid bad 
——a, to fill the cavity. In many cases this 
