[CHAPTER XXXIII.]
Liquid Air.
[Liquefaction of Gases by Northmore, 1805; Faraday, 1823; Bussy, 1824; Thilorier, 1834, and Others]—[Liquefaction of Oxygen, Nitrogen and Air by Pictet and Cailletet in 1877]—[Self-Intensification of Cold by Siemens in 1857, and Windhausen in 1870]—[Operations of Dewar, Wroblewski, and Olszewski]—[Self-Intensifying Processes of Solvay, Tripler, Lindé, Hampson, and Ostergren and Berger]—[Liquid Air Experiments and Uses].
Until quite recently the physicist divided gaseous matter into condensable vapors and permanent vapors. To-day it is known that there are no permanent gases, since all the so-called permanent gases, even to the most tenuous, such as hydrogen, may be made to assume the liquid and even the solid form. The average individual knows very little about hydrogen, but he is very well acquainted with air, and when he was told that the air that he breathes—the gentle zephyr that blows—the wind that storms from the north, or twists itself into the rage of a cyclone in Kansas—may be bound down in liquid form, and imprisoned within the limits of an open tumbler, or be bottled up in a flask or even frozen solid, he was at first impressed with the suspicion of a fairy story. Seeing is believing, however, to him, and the striking experiments from the lecture platform, the approval of the scientists, and the sensational accounts of it in the press, have not only been convincing, but have completely turned his head and made him a too willing victim of the speculator. Liquid air is a real achievement, however, and while it is astonishing to the layman, the physicist looks upon it in the most matter-of-fact way, for it is only a fulfilment of the simplest of nature’s laws, and entirely consonant with what he has been led to expect for many years.
The liquefaction of gases has engaged the attention of the scientist almost from the beginning of the century. In 1805-6 Northmore liquefied chlorine gas. This was done again in 1823 by Faraday. In 1824 Bussy condensed sulphurous acid vapors to liquid form. In 1834 Thilorier made extensive experiments and demonstrations in the liquefaction of carbonic acid gas. In 1843 Aime experimented with the liquefaction of gases by sinking them in suitable vessels to great depths in the ocean. Natterer, in 1844, greatly advanced the study of this subject by both novel methods and apparatus. Liquefaction of air was attempted as early as 1823 by Perkins, and again in 1828 by Colladon, but it was not accomplished until 1877. In this year the liquefaction of oxygen, by Pictet, of Geneva, and Cailletet, of Chatillon-sur-Seine, was independently accomplished. Pictet used a pressure of 320 atmospheres and a temperature of -140°, obtained by the evaporation of liquid sulphurous acid and liquid carbonic acid. Cailletet used a pressure of 300 atmospheres and a temperature of -29°, which latter was obtained by the evaporation of liquid sulphurous acid. In 1883 Dewar, Wroblewski and Olszewski commenced operations in this field, and greatly advanced the study of this subject. In January of 1884, Wroblewski confirmed the liquefaction of hydrogen, which had been imperfectly accomplished by Cailletet before. In the liquefaction of oxygen and nitrogen, the principal component gases of air, the liquefaction of air itself followed immediately as a matter of course.
Air has usually been held to consist of four volumes of nitrogen and one volume of oxygen, with a very small proportion of carbonic acid gas and ammonia. Recent discoveries have definitely identified new gases in it, however, chief among which is argon. For all practical purposes, however, air may be considered simply a mixture of the two gases; nitrogen, which is inert and neither maintains life nor combustion; and oxygen, which performs both of these functions in a most energetic way. Air is more dense at the surface of the earth, and becomes continually more rarified as the altitude increases, until it becomes an indefinitely tenuous ether. Light as we are accustomed to regard it, the weight of a column of air one inch square is 15 pounds, and this tenuous and unfelt covering presses upon our globe with a total weight of 300,000 million tons.
Liquid air is simply air which has been compressed and cooled to what is called its critical temperature and pressure, i. e., the temperature and pressure at which it passes into another state of matter, as from a gas to a liquid. To liquefy air it is compressed until its volume is reduced to 1⁄800, that is to say, 800 cubic feet of air are reduced to one cubic foot. This requires a pressure of 1,250 to 2,000 pounds to the square inch.
The important step in liquefying air cheaply and on a large scale was accomplished by the discovery of what is known as the self-intensifying action. This dispenses with auxiliary refrigerants, and causes the expanding gases to supply the cold required for their own liquefaction by an entirely mechanical process. It consists in compressing the air (which produces heat), then cooling it by a flowing body of water, then passing it through a long coil of pipes and expanding the cool compressed air by allowing it to escape through a valve into an expansion chamber, where its pressure falls from 1,250 pounds to 300 pounds, which produces a great degree of cold; then taking this very cold current of air back in reverse direction along the walls of the coil of pipes, and causing said returning cold air to further cool the air flowing from the compressor to the expansion tank, and finally delivering the cold return flow to the compressors and compressing it again from a lower initial point than it started with on the first round, and so continuing this cycle of circulation through the alternating compressing and cooling stages until the air condenses in liquid form in the bottom of the expansion chamber. This successive reduction of temperature by the air acting upon itself is called self-intensification of cold, and it has an analogy in the regenerative furnace, where the augmentation of heat corresponds to the augmentation of cold in the self-intensifying action.
