Point of Liquefiable Gases. 119 
which a liquid changes completely into gas, independently of 
the extent to which the pressure to which it is exposed is 
increased, but as the point at which the specific gravity 
of a liquid becomes equal, through rise of temperature, to 
that of the gas above and in contact with it. At the top 
of p. 326 of the ' Proceedings of the Koyal Society ' I 
remark : — " The critical point [of a liquid] is that point at 
which the liquid, owing to expansion, and the gas, owing 
to compression, acquire the same specific gravity, and con- 
sequently mix with one another. From the first expe- 
riment it is seen that on cooling the liquid contracts more 
rapidly than the gas, and consequently separates as a mist 
through the whole of the tube, and, from its gravity, sepa- 
rates at the lower half." M. Jamin again states that if 
a gas be compressed in a closed space, the tension of the gas 
reaches a maximum, and the gas then liquefies at its boiling- 
point under the pressure at which liquefaction takes place. 
Again let me quote : — " If the deductions from the above 
experiments be correct, it follows that that form of matter 
which we call gas may be converted into liquid by pressure 
alone ; but the meniscus will never become visible, for the 
process of change is a gradual one. To render the meniscus 
visible it is necessary to take advantage of the fact that 
liquids under such circumstances have a much greater co- 
efficient of expansion by heat, and, conversely, a much 
greater contraction on withdrawal of heat, than gases. It 
therefore becomes necessary to lower the temperature until 
the liquid by contraction acquires a specific gravity greater 
than that of its gas ; and then, and not till then, does 
the phenomenon of a meniscus become observable." Again 
(Dec. 16th, 1880), "When a mixture of liquid and gas is 
maintained at a certain volume, the expansion of the liquid, 
in raising the temperature, so long as it is possible to distinguish 
liquid from gas, points to the ultimate occupying of the space 
by liquid at temperatures above which the meniscus becomes 
invisible ; " and again, " Under such circumstances the liquid 
retains its solvent powers, while the gas is incapable of dissol- 
ving a solid." For the experimental data on which these 
assertions are based, reference must be made to the original 
paper. M. Jamin next quotes experiments of Cailletet, in 
which the latter shows that by decreasing the volume of car- 
bonic anhydride in the liquid state in contact with gaseous 
matter, oxygen or hydrogen, the meniscus gradually disappears 
when the pressure is increased sufficiently to cause the 
mixture of unliquefied carbonic anhydride and "permanent " 
