It has not yet been done; but the following telegrams, received on the 9th and 16th of April, 1883, from Cracow, by the Paris Academy of Sciences, show that chemists have come very near doing it. “Oxygen completely liquefied; the liquid colorless like carbonic acid.” “Nitrogen liquefied by explosion; liquid colorless.” Thus the two elements that make up atmospheric air have actually been liquefied, the successful operator being a Pole, Wroblewski, who had worked in the laboratory of the French chemist, Cailletet, learnt his processes, copied his apparatus, and then, while Cailletet, who owns a great iron-foundry down in Burgundy, was looking after his furnaces, went off to Poland, and quietly finished what his master had for years been trying after. Hence heart-burnings, of which more anon, when we have followed the chase up to the point where Cailletet took it up. I use this hunting metaphor, for the liquefaction of gases has been for modern chemists a continual chase, as exciting as the search for the philosopher’s stone was to the old alchemists.

Less than two hundred and fifty years ago, no one knew anything about gas of any kind. Pascal was among the first who guessed that air was “matter” like other things, and therefore pressed on the earth’s surface with a weight proportioned to its height. Torricelli had made a similar guess two years before, in 1645. But Pascal proved that these guesses were true by carrying a barometer to the top of the Puy de DÔme near Clermont. Three years after, Otto von Guerecke invented the air-pump, and showed at Magdeburg his grand experiment—eight horses pulling each way, unable to detach the two hemispheres of a big globe out of which the air had been pumped. Then Mariotte in France, and Boyle in England, formulated the “Law,” which the French call Mariotte’s, the English Boyle’s, that gases are compressible, and that their bulk diminishes in proportion to the pressure. But electricity with its wonders threw pneumatics into the background; and, till Faraday, nothing was done in the way of verifying Boyle’s Law except by Van Marum, a Haarlem chemist, who, happening to try whether the Law applied to gaseous ammonia, was astonished to find that under a pressure of six atmospheres that gas was suddenly changed into a colorless liquid. On Van Marum’s experiment Lavoisier based his famous generalisation that all bodies will take any of the three forms, solid, fluid, gaseous, according to the temperature to which they are subjected—i.e., that the densest rock is only a solidified vapor, and the lightest gas only a vaporised solid. Nothing came of it, however, till that wonderful bookbinder’s apprentice, Faraday, happened to read Mrs. Marcet’s Conversations while he was stitching it for binding, and thereby had his mind opened; and, managing to hear some of Sir H. Davy’s lectures, wrote such a good digest of them, accompanied by such a touching letter—”Do free me from a trade that I hate, and let me be your bottle-washer”—that the good-hearted Cornishman took the poor blacksmith’s son, then twenty-one years old, after eight years of book-stitching, and made him his assistant, “keeping him in his place,” nevertheless, which, for an assistant in those days, meant feeding with the servants, except by special invitation.

This was in 1823, and next year Faraday had liquefied chlorine, and soon did the same for a dozen more gases, among them protoxide of nitrogen, to liquefy which, at a temperature of fifty degrees Fahrenheit, was needed a pressure of sixty atmospheres—sixty times the pressure of the air—i.e., nine hundred pounds on every square inch. Why, the strongest boilers, with all their thickness of iron, their rivets, their careful hammering of every plate to guard against weak places, are only calculated to stand about ten atmospheres; no wonder then that Faraday, with nothing but thick glass tubes, had thirteen explosions, and that a fellow-experimenter was killed while repeating one of his experiments. However, he gave out his “Law,” that any gas may be liquefied if you put pressure enough on it. That “if” would have left matters much where they were had not Bussy, in 1824, argued: “Liquid is the middle state between gaseous and solid. Cold turns liquids into solids; therefore, probably cold will turn gases into liquids.” He proved this for sulphurous acid, by simply plunging a bottle of it in salt and ice; and it is by combining the two, cold and pressure, that all subsequent results have been attained. How to produce cold, then, became the problem; and one way is by making steam. You cannot get steam without borrowing heat from something. Water boils at two hundred and twelve degrees Fahrenheit, and then you may go on heating and heating till one thousand degrees more heat have been absorbed before steam is formed. The thermometer, meanwhile, never rises above two hundred and twelve degrees, all this extra heat becoming what is called latent, and is probably employed in keeping asunder the particles which when closer together form water. The greater the expansive force, the more heat becomes latent or used up in this way. This explains the paradox that, while the steam from a kettle-spout scalds you, you may put your hand with impunity into the jet discharged from a high-pressure engine. The high-pressure steam, expanding rapidly when it gets out of confinement, uses up all its heat (makes it all “latent”) in keeping its particles distinct. It is the same with all other vapors: in expanding they absorb heat, and, therefore, produce cold; and, therefore, as many substances turn into steam at far lower temperatures than water does, this principle of “latent heat,” invented by Black, and, after long rejection, accepted by chemists, has been very helpful in the liquefying of gases by producing cold.

The simplest ice-machine is a hermetically-sealed bottle connected with an air-pump. Exhaust the air, and the water begins to boil and to grow cold. As the air is drawn off, the water begins to freeze; and if—by an ingenious device—the steam that it generates is absorbed into a reservoir of sulphuric acid, or any other substance which has a great affinity for watery vapor, a good quantity of ice is obtained. This is the practical use of liquefying gases; naturally, they all boil at temperatures much below that of the air, in which they exist in the vaporised state that follows after boiling. Take, therefore, your liquefied gas; let it boil and give off its steam. This steam, absorbing by its expansion all the surrounding heat, may be used to make ice, to cool beer-cellars, to keep meat fresh all the way from New Zealand, or—as has been largely done at Suez—to cool the air in tropical countries. Put pressure enough on your gas to turn it into a liquid state, at the same time carrying away by a stream of water the heat that it gives off in liquefying. Let this liquid gas into a “refrigerator,” where it boils and steams, and draws out the heat; and then by a sucking-pump drive it again into the compressor, and let the same process go on ad infinitum, no fresh material being needed, nothing, in fact, but the working of the pump. Sulphurous acid is a favorite gas, ammonia is another; and—besides the above practical uses—they have been employed in a number of startling experiments.

Perhaps the strangest of these is getting a bar of ice out of a red-hot platinum crucible. The object of using platinum is simply to resist the intense heat of the furnace in which the crucible is placed. Pour in sulphurous acid and then fill up with water. The cold raised by vaporising the acid is so intense that the water will freeze into a solid mass. Indeed, the temperature sometimes goes down to more than eighty degrees below freezing. A still more striking experiment is that resulting from the liquefying of nitrous oxide—protoxide of nitrogen, or laughing-gas. This gas needs, as was said, great pressure to liquefy it at an ordinary temperature. At freezing point only a pressure of thirty atmospheres is needed to liquefy it. It then boils if exposed to the air, radiating cold—or, rather, absorbing heat—till it falls to a temperature low enough to freeze mercury. But it still, wonderful to say, retains the property which, alone of all the gases, it shares with oxygen—of increasing combustion. A match that is almost extinguished burns up again quite brightly when thrust into a bag of ordinary laughing-gas; while a bit of charcoal, with scarcely a spark left in it, glows to the intensest white heat when brought in contact with this same gas in its liquid form, so that you have the charcoal at, say, two thousand degrees Fahrenheit, and the gas at some one hundred and fifty degrees below zero. Carbonic acid gas is just the opposite of nitrous oxide, in that it quenches fire and destroys life; but, when liquefied, it develops a like intense cold. Liquefy it and collect it under pressure, in strong cast-iron vessels, and then suddenly open a tap and allow the vapor to escape. In expanding, it grows so cold—or, strictly speaking, absorbs, makes latent, so much heat—that it produces a temperature low enough to turn it into fog and then into frozen fog, or snow. This snow can be gathered in iron vessels, and mixed with either it forms the strongest freezing mixture known, turning mercury into something like lead, so that you can beat the frozen metal with wooden mallets and can mould it into medals and such-like.

Amid these and such-like curious experiments, we must not forget the “Law” that the state of a substance depends on its temperature—solid when it is frozen hard enough, liquid under sufficient pressure, gaseous when free from pressure and at a sufficiently high temperature. But though first Faraday, and then the various inventors of refrigerating-machines—CarrÉ, Tellier, Natterer, Thilorier—succeeded in liquefying so many gases, hydrogen and the two elements of the atmosphere resisted all efforts. By plunging oxygen in the sea, to the depth of a league, it was subjected to a pressure of four hundred atmospheres, but there was no sign of liquefaction. Again, Berthelot fastened a tube, strong and very narrow, and full of air, to a bulb filled with mercury. The mercury was heated until its expansion subjected the air to a pressure of seven hundred and eighty atmospheres—all that the glass could stand—but the air remained unchanged. Cailletet managed to get one thousand pressures by pumping mercury down a long, flexible steel tube upon a very strong vessel, full of air; but nothing came of it, except the bursting of the vessel, nor was there any more satisfactory result in the case of hydrogen.

One result, at any rate, was established—that there is no law of compression like that named after Boyle or Mariotte, but that every gas behaves in a way of its own, without reference to any of the others, each having its own “critical point” of temperature, at which, under a certain pressure, it is neither liquid nor gaseous, but on the border-land between the two, and will remain in this condition so long as the temperature remains the same. Hence, air being just in this state of gaseo-liquid, the first step towards liquefying it must be to lower its temperature, and so get rid of its vapor by increasing its density. The plan adopted, both by Cailletet in Paris, and by Raoul Pictet (heir of a great scientific name) in Geneva, was to lower the temperature by letting off high-pressure steam. This had been so successful in the case of carbonic acid gas as to turn the vapor into snow; and in 1877 Cailletet pumped oxygen into a glass tube, until the pressure was equal to three hundred atmospheres. He then cooled it to four degrees Fahrenheit below zero, and, opening a valve, let out a jet of gaseous vapor, which, while expanding, caused intense cold, lowering the temperature some three hundred degrees, and turning the jet of vapor into fog. Here, then, was a partial liquefaction, and the same was effected in the case of nitrogen. Pictet did much the same thing. Having set up at Geneva a great ice-works (his refrigerating agency being sulphurous acid in a boiling state), he had all the necessary apparatus, and was able to subject oxygen to a pressure of three hundred and twenty atmospheres, and by means of carbonic acid boiling in vacuo, to cool the vessel containing it down to more than two hundred degrees Fahrenheit below zero. He could not watch the condition in which the gas was; but it was probably liquefied, for, when a valve was suddenly opened, it began to bubble furiously, and rushed out in the form of steam. Pictet thought he had also succeeded in liquefying hydrogen, the foggy vapor of the jet being of a steely grey color; for hydrogen has long been suspected to be a metal, of which water is an oxide, and hydrochloric acid a chloride. Nay, some solid fragments came out with the jet of vapor, and fell like small shot on the floor, and at first the sanguine experimenter thought he had actually solidified the lightest of all known substances. This, however, was a mistake; it was some portion of his apparatus which had got melted. Neither had the liquefaction of oxygen or nitrogen been actually witnessed, though the result had been seen in the jet of foggy vapor.

Cailletet was on the point of trying his experiment over again in vacuo, so as to get a lower temperature, when the telegrams from Wroblewski showed that the Pole had got the start of him. Along with a colleague, Obszewski, Cailletet’s disloyal pupil set ethylene boiling in vacuo, and so brought the temperature down to two hundred and seventy degrees Fahrenheit below zero. This was the lowest point yet reached, and it was enough to turn oxygen into a liquid a little less dense than water, having its “critical point” at about one hundred and sixty-eight degrees Fahrenheit below zero. A few days after, nitrogen was liquefied by the same pair of experimenters, under greater atmospheric pressure at a somewhat higher temperature.

The next thing is to naturally ask: What is the use of all this? That remains to be proved. The most unlikely chemical truths have often brought about immense practical results. All that we can as yet say is, that there is now no exception to the law that matter of all kinds is capable of taking the three forms, solid, aqueous, gaseous.

The French savans are not content with saying this. They are very indignant at Wroblewski stealing Cailletet’s crown just as it was going to be placed on the Frenchman’s head. It was sharp practice, for all that a scientific discoverer has to look to is the fame which he wins among men. The Academy took no notice of the interloping Poles, but awarded to Cailletet the Lacaze Prize, their secretary, M. Dumas, then lying sick at Cannes, expressing their opinion in the last letter he ever wrote. “It is Cailletet’s apparatus,” says M. Dumas, “which enabled the others to do what he was on the point of accomplishing. He, therefore, deserves the credit of invention; the others are merely clever and successful manipulators. What has been done is a great fact in the history of science, and it will link the name of Cailletet with those of Lavoisier and Faraday,” So far M. Dumas, who might, one fancies, have said something for Pictet, only a fortnight behind Cailletet in the experiment which practically liquefied oxygen. His case is quite different from Wroblewski’s, for he and Cailletet had been working quite independently, just as Leverrier and Adams had been when both discovered the new planet Neptune. Such coincidences so often happen when the minds of men are turned to the same subject. Well, the scientific world is satisfied now that the elements of air can be liquefied; but I want to see the air itself liquefied, as what it is—a mechanical, not a chemical compound. For from such liquefaction, one foresees a great many useful results. You might carry your air about with you to the bottom of mines or up in balloons; you might even, perhaps, store up enough by-and-by to last for a voyage to the moon.—All the Year Round.