124 
POPULAR SCIENCE REVIEW. 
In 1872 the writer bought a section of rock crystal con- 
taining fluid-cavities, with the object of submitting these 
to Cagniard de la Tour’s experiment and watching the effect 
under the microscope, as it was thought that rock crystal 
would be proof against the corroding action of water. On 
warming the specimen gently over a lamp, it was found 
that a cavity previously seen to contain a liquid was appa- 
rently empty. Carefully watching the cavity under the micro- 
scope, the replacement of the liquid was noticed. Bearing in 
mind the experience of Professor Andrews, it seemed desirable 
to know at what temperature the liquid disappeared, for it was 
most probable that it had attained its critical point. The sim- 
plest mode of operating appeared to consist in immersing the 
Fig. 3. Fig. 4. 
Fig. 3 . — a shows the shape of the cavity at its upper end when the water s- 
driven out into the lower part. At b is seen the faint line indicating the 
surface of the liquid carbonic acid, as seen when warm. It should be com- 
pared with Fig. 1. 
Fig. 4 simply shows how the liquid can be driven from the lower to the upper 
end of the cavity, or vice versa , by a heated wire. 
These drawings are from the microscope, and therefore must be inverted to get 
their true position. 
slide in water of known temperature, removing, wiping it 
hastily, placing it on the microscope stage, and instantly 
examining it. A number of experiments showed that the 
liquid disappeared at 30°*8 C. to 30°*9 C., and returned instantly 
at any lower temperature. Here was most convincing evidence 
that the liquid was carbonic acid. The adjoining woodcuts, 
(figs. 1-4) will convey to the eye of the reader the general 
aspect of this cavity. 
The fact mainly regarded of interest in connection with 
this specimen was that the liquid never expanded by heat ; thus 
