Van Marum, a philosopher and chemist of Harlem, is celebrated as the constructor of an electric machine, the largest known, but he is more justly celebrated for having been the first to liquefy a gas. Wishing to know if ammonia would obey Mariotte’s law, he compressed it. Under a pressure of six atmospheres it changed quickly to a transparent liquid. Van Marum did not foresee the consequences of his experiments, and is honored only as being the first successful performer of the experiment. But Lavoisier, whose keener mind grasped all that these results implied, did not hesitate to declare the general law that all substances were capable of existing in three different states, and he illustrated his belief most forcibly. “Let us consider for a moment what would happen to the different substances which form the earth, if the temperature should be quickly changed. Let us suppose that the earth were suddenly placed in a region where the temperature would be much above that of boiling water; soon the air, all liquids which can be vaporized at a temperature near that of boiling water, and many metallic substances even, would expand, be transformed into air-like fluids, and form part of the atmosphere.
“On the contrary, if the earth should be suddenly placed in a very cold temperature, for example, that of Jupiter or Saturn, the water of our rivers and seas, and, probably, the greatest number of liquids which we know would become solid.”
“Air,” according to this supposition, or at least a part of the air-like substances which compose it, “would doubtless cease to exist in its present form; it would be changed to a liquid state, and this change would produce new liquids of which we know nothing.”
Lavoisier was mistaken about the temperature of Jupiter and Saturn, but was right in his supposition that air would become a liquid; however, as experiment did not prove the theory, the prediction was forgotten and the question dropped. It slept a long time, for it was not until 1823 that it was revived by Faraday. The first experiments of this great philosopher were on this subject. He was but twenty-two when he made his first discovery, the liquefaction of chlorine. The details of this experiment have been told by Tyndall. It is well known that when chlorine gas and cold water are united, crystals are formed which contain to every molecule of chlorine ten molecules of water. Faraday put some of these into a closed tube and heated them until two separate liquids appeared; one was water, the other floated on the surface of the water, and a certain professor of Paris declared that it could be nothing but oil carelessly left in the vessel. Faraday having opened the tube, found that this substance began to boil, and then changed with an explosion into a green gas. It was chlorine. Faraday, who was quick-tempered, immediately took his revenge on the professor, to whom he wrote: “You will be pleased to know, sir, that the oil left by carelessness in my apparatus was nothing less than liquefied chlorine.”
This first success decided the career of the young chemist. He announced that all gases could be reduced to this state if subjected to a sufficient pressure, and he undertook a series of experiments, of which the success was doubtful, but the danger certain. He operated in this way: He took a thick glass tube in the form of an inverted U; one branch was left empty, in the other the materials for producing the gas to be studied were placed and the whole closed. Obliged to gather in the empty branch, the gas continually increased in pressure, and there were two possible results to the experiment; either the gas would not change its state, and the pressure would increase until the vessel broke, or when a certain limit of pressure was reached, then the liquid would appear and would continue to accumulate as long as the gas was disengaged. A dozen gases were reduced in this way; among them were the following, which we shall need: Ammonia, sulphurous acid, carbonic acid, and protoxide of nitrogen, which at a temperature of ten degrees required a pressure equal to sixty atmospheres.
This pressure leaves no doubt about the danger which one runs in carrying on such researches. If we remember that steam boilers generally support a pressure of no more than ten atmospheres, if we recall the number and the horror of their explosions we can hardly understand how a simple glass tube could resist a pressure five or six times as great. When a gas reaches the point of liquefaction, then the pressure ceases to increase, but if it does not change from that condition the pressure increases until an explosion necessarily occurs, and the debris of the vessel is scattered as powder scatters the fragments of a shell. In the course of Faraday’s researches he had thirty explosions. They did not stop him, but it is easy to see that they did not encourage others.
Happily there is a less dangerous method of reaching the same result, it is to freeze the gas. In the same way that the vapor of water is condensed when the temperature is lowered, so gases, which are really vapors, will yield to sufficient cold. In 1824, Bussy succeeded in condensing sulphurous acid gas. The gas was introduced into a balloon, which was plunged into a freezing mixture of ice and salt. The gas was liquefied and could be preserved indefinitely, if the balloon were enclosed in an enamel vessel. In heating, it gave off vapors which, by their pressure kept the remainder of the fluid, providing the glass was strong enough. Thus, in two ways, by cold and by pressure, and still better, by both combined, it is possible to liquefy a large number of gases.
When water is heated, it remains immovable up to 100 degrees Centigrade, but then it is changed into vapor, or boils. This boiling is characterized by a peculiar feature, the temperature remains fixed at 100 degrees. It must be concluded, therefore, that the heat produced by the furnace and absorbed by the liquid is simply used in transforming the water into vapor. This fact was first discovered by the English philosopher, Black, who, not being able to explain the phenomenon, was content to demonstrate it and to speak of the heat as latent. He saw that it took five and a half times as long to change water into vapor as to heat it from zero to 100 degrees, and that consequently it must require five and a half times as much heat to work the change. Such is the law of boiling in the air, but let us see what it is in a vacuum.
It is clear that the pressure of the atmosphere on water is a hindrance to its expansion into vapor, and that this hindrance increases or diminishes with the pressure. In a vacuum, of course, the liquid is free from the pressure, so that boiling ought to take place at a lower temperature.
And experiment teaches that this is the case; water boils at a temperature of 82° or 65°, as the pressure is reduced to one half or a quarter of an atmosphere, it boils at zero, and even below, in a vacuum. And we reach this remarkable result, that the boiling and freezing points unite, and that ice is formed while vapor is set free. But, although the boiling is advanced, although it takes place at zero instead of at 100 degrees, although the vapor is cold instead of hot, and the change takes place in a vacuum instead of in the air, it is a general law that a large quantity of heat is used, becomes latent, and enters into the formation of vapor.