MINERAL CAVITIES AND THEIR CONTENTS. 
125 
it differs from that in cavities examined by any previous 
observer. 
The following observation of Thilorier explains the reason : — 
44 When a tube containing liquid carbonic acid is one-third 
full, at 0° C., it constitutes a retrograde thermometer , in which 
increase of temperature is shown by diminished volume, conse- 
quent on the vaporization of the liquid, and 
vice versa ; while if the tube be two-thirds full, 
a normal thermometer of great sensitiveness is 
the result, the liquid expanding by heat in this 
case” ( 44 Ann. Chim. Phys.” [2], lx. 249). 
It will be seen that in the cavity the pro- 
portion of liquid to gas resembles that in a 
retrograde thermometer, and furthermore the 
approach of a warm wire causes the liquid 
gradually to diminish. Especially rapid is 
this action as the critical point is approached, 
and at the same time the line of demarcation 
between liquid and gas gets fainter, and it atVs^c/x ioTiam 
loses its curvature. (See fig. 3.) Heat expands the 
There was also noticed a faint flickering liquid, a, gaseous 
shadow in the point of the cavity when the carbonic 
liquid was about to condense. This is doubt- £cid. 
less an effect corresponding to the strise noticed 
by Professor Andrews. The liquid carbonic acid was always 
found floating on water if this latter substance happened to 
Fig. 5. 
Fig. 6. 
Water Liquid Gas Liquid Wat 
Carbonic acid Carbonic acid = 250 
Fig. 7. 
Spherules of gas Gaseous carbonic acid 
Liquid Liquid spherules 
Carbonic acid 
liquid cayity in TouEMAiiNE x 250 diameters. 
be present, and this is quite in accordance with the specific 
gravity as estimated by Thilorier, which at 0° C. is 0*83 and at 
30° C. is 0-6. 
