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POPULAR SCIENCE REVIEW. 
A cavity of quite a different description is that found in a 
topaz (fig. 5). It contains only a trace of water, and is at 
ordinary temperatures filled to the extent of two-thirds its 
capacity by liquid carbonic acid. Rise of temperature ex- 
pands this liquid until it entirely fills up the space occupied 
by gas, after which expansion the critical point is very easily 
Fig. 8. Fig. 9. 
Fig. 8 shows the proportions of liquid and gaseous carbonic acid and water. 
Fig. 9 shows the boiling motion when it has nearly terminated. 
attained. A very curious appearance, resembling ebullition, 
follows as the gas returns to liquid, the explanation of which is as 
follows : — When the gas is chilled, a sort of mist forms through- 
out the space ; the individual spherules of this mist grow so large 
that they begin to touch each other, to coalesce, and to gravi- 
tate. They of course at the same time entangle gas, and as 
Fig. 10. 
they descend to the lower part of the cavity the spherules of 
gas (bubbles) take an opposite direction ; consequently when a 
portion of the liquid has collected at the lower end and gas at 
the upper, there are showers of liquid descending into, and 
streams of bubbles rising out of the liquid. In two or three 
