G-ASEOUS AND LIQUID STATES OF MATTEE. 
583 
The curve representing the results at 21 0, 5 agrees in general form with that for 
13°‘l, as shown in the above figure. At 13°‘l, under a pressure of about 49 atmospheres, 
the volume of carbonic acid is little more than three-fifths of that which a perfect gas 
would occupy under the same conditions. After liquefaction carbonic acid yields to 
pressure much more than ordinary liquids ; and the compressibility appears to diminish 
as the pressure increases. The high rate of expansion by heat of liquid carbonic acid, 
first noticed by Thilorier, is fully confirmed by this investigation. 
The next series of experiments was made at the temperature of 31°T, or 0 O- 2 above 
the point at which, by compression alone, carbonic acid is capable of assuming visibly the 
liquid form. Since I first announced this fact in 1863, 1 have made careful experiments 
to fix precisely the temperature of this critical point in the case of carbonic acid. It 
was found in three trials to be 30 o, 92 C., or 87 0, 7 Fahr. Although for a few degrees 
above this temperature a rapid fall takes place from increase of pressure, when the gas 
is reduced to the volume at which it might be expected to liquefy, no separation of the 
carbonic acid into two distinct conditions of matter occurs, so far as any indication of 
such a separation is afforded by the action of light. By varying the pressure or tempe- 
rature, but always keeping the latter above 30 o- 92, the great changes of density which, 
occur about this point produce the flickering movements I formerly described, resembling 
in an exaggerated form the appearances exhibited during the mixture of liquids of dif- 
ferent densities, or when columns of heated air ascend through colder strata. It is easy 
so to adjust the pressure that one-half of the tube shall be filled with uncondensed gas 
and one-half with the condensed liquid. Below the critical temperature this distinction 
is easily seen to have taken place, from the visible surface of demarcation between the 
liquid and gas, and from the shifting at the same surface of the image of any perpendi- 
4 i 2 
