Supposing that we fill a bronze vessel of very thick sides with water, close it with a lid and fit into it a valve loaded with lead. Place this in a furnace whose temperature has been raised to, say, 230 degrees. The water will reach this temperature, and vapor will accumulate until it reaches a pressure equal to more than twenty-seven atmospheres.
Let us now open the valve, the vapor will escape, and as it carries with it the heat necessary for its expansion, the temperature of the water will gradually fall until it reaches 100 degrees, after which the boiling will continue slowly and regularly; thus the water has been cooled and is kept below the temperature of its surrounding wall because it must absorb the extra heat which is required to change it to vapor. This apparatus is called Papin’s digester.
There is a similar experiment, but performed in a vacuum at the ordinary temperature. Put some water into a closed decanter which is connected by a tube with an air pump. As soon as a vacuum is produced the water begins to boil and to freeze, for the vapor can only be formed by borrowing heat, and there is nothing to take it from but the water itself, which soon reaches zero and is frozen. This apparatus makes a very simple ice house, as useful as convenient, and it proves, first, that boiling takes place at the lowest temperatures providing the pressure is sufficiently diminished; secondly, that it is always accompanied by a loss of heat; and thirdly, that it lowers the temperature of the liquid below that of the surrounding envelope, and the more as the vacuum is more complete.
Just as opening the valve lets the vapor accumulated above the water in Papin’s digester escape, and causes a fall in the temperature, so, by opening the reservoirs in which one has confined a liquefied gas, one sees it fall back to the boiling point. For example, take the liquid obtained from the compression of sulphurous acid gas. As soon as the reservoir containing it is opened the liquid begins to boil, and a vapor is formed, it is the gas which re-forms. It absorbs the latent heat necessary, taking it from exterior objects by radiation from the liquid itself, from the vessel which holds it, and from the materials into which it has been placed. It cools these until the point at which sulphurous acid gas boils is reached, twelve degrees below zero; then the liquid remains balanced between the radiation which tends to heat it and vaporization, which cools it. The final result is that the temperature is lowered and remains fixed at twelve degrees below zero. This is not all: just as the boiling point of water is lowered below zero in a vacuum, in the same way that of sulphurous acid gas falls below twelve degrees. Bussy brought it down to sixty-eight, where it remained; not only water, but mercury may be frozen by this means.
Finally, the boiling of liquefied gases will freeze all neighboring substances, and the greatest cold which one could obtain is produced by their boiling in a vacuum. This property of sulphurous acid was discovered in a still greater degree in protoxide of nitrogen, which was changed into a liquid at a temperature of 0 degrees, and under a pressure of thirty atmospheres. If allowed to boil in a vacuum, a temperature of one hundred and ten degrees below zero was obtained. When science has sown trade reaps the harvest; since by allowing liquefied gases to boil, a temperature of one hundred and ten degrees below zero can be obtained, and since the vapors which they give off carry away an enormous amount of heat from the surrounding bodies, it is possible by means of this cold produced to freeze water, make cold drinks, solidify mercury, cool cellars, prevent food from decay, and to do many other things of similar nature. A new art became possible, that of making cold. To-day it is at the height of success. It is founded on this general principle: to liquefy the gas by means of pressure, taking care that it does not become heated, to introduce it into a freezer, where it is allowed to boil, and from which it absorbs the heat, to carry off the gas and introduce it again into the vessel, where it will by pressure be liquefied. The action is constant, the same gas acts indefinitely, and there is no other expense than that which is caused by running the pumps. In spite of these fine results and the extraordinary efforts put forth, the end was not attained. To be sure, some gases had yielded, but still there was a large number which resisted every effort. Was it necessary to give up the idea that the law of liquefaction of gases was general, or was it true that the exceptions were only the results of insufficient means? Faraday had never varied in his belief. One easily returns to the affections of his youth, and he believed that the time had come for making fresh efforts to prove his theory. After a rest of twenty-two years he determined to again take up the liquefaction of the rebellious gases. Means were not wanting. Thilorier had taught him how to solidify easily large masses of carbonic acid, and by mixing this solid with ether make a powerful freezing mixture; protoxide of nitrogen could be prepared with the same ease and abundance, and would boil regularly in a vacuum at a temperature of one hundred and twenty degrees below zero. Thus he was able to secure a degree of cold before unknown. For compression, he had a pump formed of two parts; one took the gas at its generation, and accumulated it in a reservoir under a pressure of fifteen atmospheres; the second part then received it; here it was subjected to a much greater pressure in a strong glass vessel which was plunged into carbonic acid or protoxide of nitrogen. Cold and pressure were thus combined. At that time nothing more could be done; fortunately this was enough to subdue most gases. Faraday had the satisfaction of liquefying nearly all gases, and of extending the law which he had announced, but still six, only six, refused to give up; among them were marsh gas, oxygen, nitrogen, and hydrogen. Science is a battle which must be continually renewed; the more the gases resisted, the greater the efforts made to conquer them. At first, new and energetic means of pressure were invented. Aimé, a professor in Algiers, secured a pressure of four hundred atmospheres, without result. M. Cailletet used a hydraulic press which exerted a force equal to seven hundred atmospheres, and afterward increased this to one thousand atmospheres, but still the gas resisted. At last it was found that pressure alone, however enormous it might be, could not liquefy the gases.
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.
Wroblewski and his colleague, Olszewski, had boiled ethylene, a gas similar to that used for heating purposes, in a vacuum. The temperature fell to one hundred and fifty degrees below zero. It was the greatest degree of cold yet obtained, and was sufficient. The success was complete. The oxygen, previously compressed in a glass tube, became a fixed liquid. It was like the others, in the form of a colorless and transparent liquid, like water, but of a little less density. Its critical point was at one hundred and thirteen degrees below zero, forming itself below, never above, this temperature, and boiling rapidly at a temperature of one hundred and eighty-six degrees below zero. A few days after this the two Polish professors succeeded, in the same way, in liquefying nitrogen.
But if the question was settled for air was it also for nitrogen? M. Pictet, in his experiment, had used a weight of three hundred and twenty atmospheres, and cold of one hundred and forty degrees below zero. When he opened the reservoir a jet of gas, mingled with mist of steel gray color, burst forth. At the beginning of the experiment, solid fragments accompanied the jet; these fell to the floor with a sound like that of grains of lead. Naturally, M. Pictet thought that he had not only liquefied, but even solidified hydrogen, but unfortunately the experiment was not wholly satisfactory. For perfect success still more acute cold was needed, and here was oxygen and nitrogen to get it from. Nitrogen, the most refractory, was taken, and a degree of cold undreamed of before, attained; in the open air it reached one hundred and ninety-four degrees below zero, and in a vacuum two hundred and thirteen degrees below. These temperatures were so low that it was necessary to invent new methods for measuring them. A mercury thermometer was useless, because it froze at forty degrees, and alcohol because it became a solid at one hundred and thirty degrees. No liquid is able to resist such temperatures, so electric, or hydrogen thermometers, were employed.