MINERAL CAVITIES AND THEIR CONTENTS. 
127 
seconds the movements have ceased. This ebullition is illus- 
trated by a drawing of a cavity in another topaz, which contains 
at ordinary temperatures about equal volumes of water, gaseous 
carbonic acid, and liquid carbonic acid (figs. 8 and 9). A 
similar cavity as to its contents, but being exceedingly shallow, 
and having the bubbles within it much flattened, is that exist- 
ing in a tonrmaline (figs. 6 and 7). The drawing, fig. 10, 
represents a cavity seen in a specimen of quartz ; the contents 
are undergoing apparent boiling. The conditions favouring 
this singular mode of condensation seem to be, first, that the 
greater part of the carbonic acid shall be in the liquefied 
state at ordinary temperatures, so that the liquid expands 
greatly on approaching the critical point ; secondly, that the 
cooling shall be sudden. This seeming ebullition is the 
only test which as a general rule is significant of the liquid 
having passed its critical point, for we cannot tell after the 
substance has expanded so as to entirely fill the cavity, whether 
it be in the gaseous or the liquid state. 
The following table shows several variations noticed in the 
critical point of carbonic acid existing in various minerals : — 
Critical point. 
Topaz 
28° C. = 82°-4 F. 
Topaz 
28° and 26°-5 C. = 82°-4 F. and 79°-7 F. 
Topaz 
27°*55 C. = 81°*5 F. 
Tourmaline 
27°-27 C. 
Tourmaline 
26° -9 C. 
Sapphire . 
between 30°*5 and 31° C. 
Sapphire . 
between 25°*5 and 26° C. 
Sapphire . 
29°-o C. = 85°*1 F. 
Hock crystal 
• • 30*95 0. 1 87-71 F. 
Rock crystal 
30°-9oC. J 
Rock crystal 
32 o, 0 C. = 90 o, 5 F. 
Rock crystal 
33°*7 C. 
Rock crystal 
29° C. = 84°*2 F. 
Rock crystal 
30°*95 C. 
Beryl 
30°*92 C. = 87°'G F. 
Rock crystal 
21° C. = 69° *8 F. 
It is noticeable that in the sapphires, topazes, and tourma- 
lines the critical point is lowered a few degrees, while in one 
specimen of rock crystal the temperature is nearly 10° C. or 1 8° F. 
below what it should be. Fortunately, Professor Andrews has 
shown that the critical point undergoes such a change when traces 
of any incondensable gas are present. Carbonic acid containing 
15 per cent, by volume of nitrogen has a critical point of about 
21° C., and it is not at all unlikely that this is the gas here 
present, since Sir Humphry Davy detected it in the fluid-cavities 
