An English philosopher, called Andrews, put a new face on matters. He took carbonic acid gas at a temperature of about thirteen degrees and compressed it. The gas began to diminish in volume, and under a pressure of fifty atmospheres was suddenly liquefied, taking quickly a very great density, and falling to the bottom of the vessel, where it remained separated from its vapor by a surface as plainly marked as that which marks water and air. Andrews afterward tried the same experiment at a higher temperature, about twenty-one degrees. The same results were produced with but one difference: the liquefaction was less sudden. At a temperature of thirty-two degrees, instead of a separate and distinct liquid, undulating striæ appeared as the only signs of a change in condition which was not completed. Finally, at a temperature of above thirty-two degrees there was neither striæ nor liquefaction, but still it seemed as if a trace was preserved, for under certain pressure the density increased more quickly, and the volume diminished more rapidly. Thirty-two degrees is then the limit, a point between the degrees which permit and which prevent liquefaction. It is the _critical point_ which marks the separation between two very different conditions of a substance; below, we have a liquid; above, there is no change in appearance, but there enters a new condition, whose characteristics I will describe.
Generally a liquid is more dense than its vapor; for this reason it falls to the bottom, and the two are separated by a level surface. But supposing that we heat the vessel which contains them. The liquid expands little by little, until it equals, or even surpasses, the expansion of the gas, so that an equal volume weighs less and less. On the other hand, a continually increasing quantity of vapor is formed, accumulates at the top of the vessel, and becomes constantly heavier. Now, if the density of the vapor increases, or if that of the liquid diminishes under the right temperature, the two densities become equal. Then there is no longer a reason for the liquid falling, the vapor rising, or for a surface of separation. The two are mingled. Neither are they any longer distinguished by their different degrees of heat. When this critical point is reached, it is impossible to tell whether it is liquid or gas, since in either state it has the same density, the same heat, the same appearance, the same properties. This is a new state, a gaseous liquid state. The discovery of these properties showed why all the attempts to liquefy air had been useless. At an ordinary temperature the gas is in a gaseous liquid condition. Liquefaction can take place only when the liquid is separated from the vapor by its own greater density. The next step was therefore to lower the temperature below that of the critical point. This was understood and carried out about the same time by MM. Cailletet and Raoul Pictet. On the 2nd of December, 1877, M. Cailletet subjected oxygen in a glass tube to a pressure of three hundred atmospheres, and reduced its temperature to twenty-nine degrees below zero. The gas did not change in appearance, and was in all probability in the gaseous liquid condition. Nothing but more cold was wanting to liquefy it. The valve was turned, the gas escaped, and the temperature fell two hundred degrees, and the characteristic whitish mist was seen. Oxygen had been liquefied, perhaps solidified. The same result was reached with nitrogen, but nothing was done with hydrogen. While M. Cailletet performed this decisive experiment at Paris, M. Raoul Pictet achieved the same at Geneva. He had at his command all necessary materials, so that he subjected the oxygen to a pressure of three hundred and twenty atmospheres, and to a temperature of one hundred and forty degrees below zero. In this condition the gas was probably below the critical point, and when the reservoir was opened suddenly it began to boil and was thrown in every direction. M. Pictet believed that he liquefied, and even more, had solidified hydrogen, but he was doubtless mistaken. These results, however, were not satisfactory. M. Cailletet was preparing a new experiment when the Academy received the two telegrams given at the beginning of this article.