GASEOUS AND LIQUID STATES UNDER VARIOUS CONDITIONS. 
55 
I was long perplexed with anomalous results, the carbonic acid sometimes liquefying 
by the application of pressure at temperatures above 20°, at other times refusing to 
liquefy at temperatures several degrees below that point. After many trials these 
irregularities were at last traced to the gaseous mixture having separated into two 
portions, one rich and the other poor in carbonic acid, when the pressure was reduced 
after liquefaction so as to convert the whole mixture into the gaseous state. In this 
case the lower end of the tube, which had been before occupied by the liquid, 
contained a large excess of carbonic acid, particularly if the tube had been previously 
exposed to —10° so as to liquefy nearly the whole of the carbonic acid in the mixture. 
On reducing the pressure so as to restore the carbonic acid to the gaseous state, and 
raising the temperature to 26°, I have succeeded in liquefying the carbonic acid by 
the application of pressure alone at this temperature, provided the experiment was 
performed without loss of time. But, on allowing the mixture to remain for some 
time in the gaseous state, diffusion gradually occurred, and the temperature at which 
liquefaction was possible underwent a corresponding reduction. The diffusion was not 
complete for several hours, after which no liquid was formed by the application of 
pressure till the temperature was reduced to 14°. The true critical point for this 
mixture was therefore just below 14°, and when liquid was formed by pressure at 
higher temperatures it arose from carbonic acid gas being present in excess in the 
lower end of the tube. 
Advantage was taken of this mode of separating a mixture of nitrogen and carbonic 
acid to ascertain whether any or what change of volume occurs in the diffusion of 
these gases at high pressures. For this purpose the gases were left for about twenty 
hours under nearly the same pressure as that at which the observations were subse¬ 
quently made. The volume of the mixture was then carefully read at a fixed 
temperature and pressure. The mixed gases were now exposed to high pressure at 
the temperature of —12° so as to liquefy the carbonic acid. They were again restored 
to the original pressure and temperature, and the volume was read. It was always 
found to have increased, usually about one-sixtieth part. On allowing the whole to 
remain at rest, and preserving the temperature and pressure steadily the same, the 
original volume was gradually restored by the expansion which occurred as the gases 
slowly diffused into one another. In an hour and a half the mixture had recovered 
four-fifths of the first contraction. On the other hand, the application of strong 
pressure at temperatures at which no liquefaction occurred did not produce any 
change in the volume of the mixture when the original temperature and pressure were 
restored. The following are the details of three sets of observations :— 
At 8 0, 5, and under a pressure of 46‘4 atmospheres, the volume of the mixed gases, 
after complete diffusion for many hours, was found to be 162‘2. After liquefying the 
carbonic acid by strong pressure at —12°, and again restoring the temperature to 8 0, 5 
and the pressure to 46’4, the volume of the mixed gas was reduced to 159'5, a con¬ 
traction of 27 volumes having taken place from the separation of the diffused gases. 
