Intelligence and Miscellaneous Articles. 73 
limit, it becomes and remains equal to the latter : this is the 
critical point. 
(4) At the critical point and beyond it the liquid is mixed and 
blended with its saturated vapour. 
(5) At the critical point and beyond it there is no longer any 
latent heat. 
(6) At the critical point and beyond it the liquid and gaseous 
states are blended. 
From my point of view it is seen that all the laws of the 
formation and condensation of vapours are maintained, and that 
the exception of the critical point is explained by the equalization 
of the densities. We shall see that the same theory accounts for 
facts hitherto incomprehensible, and enables us to predict others 
which are not without interest. M. Cailletet in 1880 * made the 
following experiment, which has not received the attention it 
deserves, and has been forgotten because it was not understood : — 
Having compressed in his apparatus a mixture of 1 part air and 5 
parts carbonic acid, M. Cailletet first saw the latter gas assume the 
liquid state under a moderate pressure ; then, without changing 
the temperature, but raising the pressure to 150 or 200 at- 
mospheres, he saw the whole of the liquid which had been formed 
disappear. One would say that the increase of pressure gives rise 
to a critical point, like the elevation of the temperatures, which is 
scarcely admissible. The theory which I have just established 
accounts for this curious phenomenon very easily as follows : — 
By a moderate pressure carbonic acid is brought to its con- 
densing-point and is at first partially liquefied. If the reduction 
of volume is continued, the pressure of the carbonic acid is no 
longer augmented, because it had already attained its maximum ; 
but that of the air continues to increase indefinitely, and with it 
the total density of the gaseous atmosphere. This density soon 
becomes equal to that of the liquid already formed, which then 
does not remain at the bottom of the vessel, but diffuses into the 
whole space, having, according to the principle of Archimedes, 
lost all its weight. 
It is by the accuracy of its previsions that the truth of a theory- 
is judged. Now, if it be true that the disappearance of the 
liquefied acid is due to the equalization of the densities, it will be 
retarded by substituting for the air of the mixture a gas of less 
density, hydrogen. 
I forwarded this conclusion to M. Cailletet, who hastened to 
submit it to experiment. He made two mixtures, the first 
containing 5 volumes of carbonic acid to 1 volume of air, and the 
second containing 5 volumes of carbonic acid to 1 volume of 
hydrogen. In both cases he obtained the liquefaction of the car- 
bonic acid under moderate, and its total disappearance with more 
powerful pressures. In both cases, first the meniscus was effaced ; 
this was at the moment when the densities of the liquid and 
• Comptes Re.ndus, Feb. 2, 1880. 
Phil. Mag. S. 5. Vol. 16. No. 97. July 1883. G 
