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.

This principle of self-intensification was first announced by Prof. C. W. Siemens in the provisional specification of his British patent No. 2,064, of 1857, but it does not seem at that time to have been carried out with any practical result. The first embodiment of the principle in a refrigerating apparatus is by Windhausen—United States patent No. 101,198, March 22, 1870. Solvay, in British patent No. 13,466, of 1885, gave further development to the idea, and following him came the operations of Prof. Tripler, who was the first to liquefy large quantities of air and to introduce it to the American people. LindÉ, Hampson and Ostergren and Berger are more recent operators in this field of self-intensification, and LindÉ’s British patent, No. 12,528, of 1895, may be regarded as a representative exposition of the principle. A simplified form of the LindÉ apparatus is seen in Fig. 300. C is an air compressing pump, whose plunger descending compresses the air and forces it out through valve I, pipe 2, and coil 3. The coil 3 is immersed in a flowing body of water in the condenser W, the water entering at Y and passing out at Z. The cold compressed air then passes through pipes 4 and 5, pipe 5 being arranged concentrically within a larger coil 7. The cold air flowing down pipe 5 escapes through a valve adjusted by handle 6, into the subjacent chamber L, and expanding to a larger volume, produces a great degree of cold; this cold expanded air then passing up the larger and outer pipe 7 flows back over the incoming stream of air in pipe 5, chilling it still lower than the condenser W did, and this cold return flow then passing from the top of coil 7 descends through pipe 8 to the compressing pump C, and as its piston rises, it enters the pump through the inwardly opening valve 9, and here it undergoes another compression and circuit through the pipes 2, 3, 4, 5, but it is compressed on its second round of travel at a lower temperature than it had initially, and so this circulation of air going to the chamber L, expanding, and returning over the inlet flow pipe 5, successively cooling the latter and also successively entering the compressor at a continually lower temperature at each cycle of circulation, finally issues through the valve at the lower end of pipe 5, and expands to such a low temperature that it condenses in chamber L in liquid form. Fresh accessions of air are furnished to the apparatus through valve 10 as fast as the air is liquefied. The inlet flow to the liquefying chamber is shown by the full line arrows, and the return flow to the compressor by the dotted arrows, and the explanation of the term self-intensification is to be found in the cooling of the incoming air in pipe 5 by the outflowing air in the surrounding pipe 7, and the repeated reductions of temperature at which the air is returned to the compressor.

In Fig. 301 is shown the liquefier of a modern liquid air plant, in[451]
[452] which liquid air is being drawn into a pail from the liquefier. Liquid air evaporates very rapidly, and produces the intense cold of 312° below zero. There is no known way to preserve it beyond a limited time, for, if put in strong, tightly closed vessels, it would soon absorb enough heat to vaporize, and in time would acquire a tension of 12,000 pounds per square inch, and would burst the vessel with a disastrous explosion. If left exposed to the air, which is the only safe way to transport it, it is quickly dissipated. A shipment of eight gallons from New York to Washington for lecture purposes shrunk to three gallons in two days’ time. It may usually be kept longer than this, however, as the jarring of a railway train promotes its evaporation and loss. A small quantity, such as a half pint, will boil away in twenty-five to thirty minutes. The only way to preserve it for any length of time is to surround it with a heat-excluding jacket. The simplest and most effective means for doing this in the laboratory is to surround it with a vacuum. Fig. 302 shows a specially devised vessel for the commercial transportation of liquid air. A double walled globular vessel has between its walls air spaces and non-conducting packing. The liquid air in the interior chamber vaporizes gradually, and escaping through the outwardly opening valve at the top, expands around the air space surrounding the inner vessel. From this space it reaches the outer air by a valve at the bottom of the outer vessel. The liquid air in evaporating is thus carried around the body of liquid air in the center, and surrounding it with an intensely cold envelope, prevents the transmission of heat to the inner vessel. To withdraw the liquid air, a pipette or so-called siphon tube, shown in detached view, is substituted for the valve at the top.

As to the uses of liquid air it may be said that up to the present time it has attained little or no practical application. There are two principal ways in which it may be utilized; one is to employ its enormous expansive force to produce mechanical power, and the other is as a refrigerant. As a means for obtaining motive power it is a fallacy to suppose that any more power can be obtained from its expansion than was originally required to make it. It is like a resilient spring in this respect, that it can give out no more power than was required to compress it. In some special applications, however, as for propelling torpedoes, where its cost is entirely subordinate to effective results, it might prove to be of value. For blasting purposes also it presents the promise of possible utilization. As a refrigerant for commercial purposes, and for supplying a dry, cool temperature to the sick room, and for the preparation of chemicals requiring a low temperature to manufacture, it might find useful application. Inasmuch as the nitrogen of liquid air evaporates first, and leaves nearly pure liquid oxygen, it may also be employed as a means for producing and applying oxygen. Good illustration of this is given in Fig. 303, in which at 1 is shown a vessel filled with liquid air. The gas first evaporating is nitrogen, and a lighted match applied to the surface of the liquid is quickly extinguished, since nitrogen does not support combustion. As the level of the liquid falls by evaporation, the remaining portions become richer in oxygen and poorer in nitrogen, and nitrous oxide gas is then given off, which supports combustion as seen at 2; and when the last portions of the liquid are being evaporated, as at 3, it is practically pure oxygen, which gives a brilliant combustion of a carbon pencil, or even of a steel spring when the latter is heated red hot. Already Prof. Pictet has formulated a plan for the commercial production and separation of the ingredients of liquid air—the nitrogen, carbonic acid, and oxygen being separated by their different evaporating temperatures with a view to applying them to various industrial uses. All of the commercial applications of liquid air, however, depend upon its cost of production, which seems at present an uncertain factor. According to the claims of some it may be produced at a cost of a few cents a gallon. More conservative physicists say that it costs $5 a gallon.

However this may be, the phenomena which it presents are both interesting and instructive. In Figs. 304 and 305 are shown some of the experiments. At No. 1 a test tube containing liquid air, from which the nitrogen has escaped, is strongly attracted by an electro-magnet, showing the magnetic quality of oxygen. At No. 2 is shown the combustion of a heated piece of steel in liquid air, which has become rich in oxygen by the evaporation of the nitrogen. At No. 3 a tin dipper, which has been immersed in liquid air, has become so cold and crystalline that it breaks like glass when dropped. At No. 4 liquid air imprisoned in a tube and tightly corked up, blows the stopper out in a few minutes with explosive effect. At No. 5 a piece of paper saturated with liquid air burns with great energy, and at No. 6 a piece of sponge or raw cotton similarly saturated explodes when ignited. At No. 7 a rubber ball floated on liquid air in a tumbler is frozen so hard that when dropped it flies into fragments like a glass ball. The white, snow-like vapor seen falling over the edges of the tumbler is intensely cold and heavier than ordinary air. At No. 8 is illustrated the preservation of liquid air by surrounding it with a vacuum in a Dewar bulb. At No. 9 a flask of liquid air is made to boil by the mere heat of the hand. A more striking experiment still of the same kind is to place a tea kettle containing liquid air on a block of ice. The block of ice is relatively so much hotter than the liquid air that the liquid air in the kettle is made to boil. At No. 10, Fig. 305, a heavy weight is suspended by a link composed of a bar of mercury frozen solid in liquid air. So hard is the mercury frozen that a hammer made of it will drive a tenpenny nail up to its head in a pine board. In No. 11 a layer of liquid air on water at first floats because it is lighter than water. As the lighter nitrogen evaporates, the heavier oxygen sinks in drops through the water. At No. 12 a tumbler of whiskey is frozen solid by immersing a tube containing liquid air in it. The frozen block of whiskey with the cavity formed by the tube is shown on the left. It is a whiskey tumbler made out of whiskey. A more sensational experiment is to substitute a tapering tin cup for the tube, then fill it with liquid air and immerse it in water. In a few minutes the tapering tin cup has frozen on its outer walls a tumbler of ice. This may be carefully removed, and the ice tumbler is then filled with liquid air rich in oxygen, which, by maintaining the cold of the ice tumbler, keeps it from melting. A carbon pencil or a steel spring heated to redness will now, if dipped in the liquid oxygen in the ice tumbler, burn with vehement brilliancy and beautiful scintillations, involving the anomalous conditions of a white hot heat and active combustion in the center of a tumbler of ice, without melting the tumbler. In experiment 13, Fig. 305, a jet of carbonic acid gas directed into a dish floating in a glass of liquid air is immediately frozen into minute flakes, producing a miniature snow storm of carbonic acid. In experiment 14 an electric light carbon heated to a red heat at its tip, is plunged vertically into a deep glass of liquid oxygen. A most singular combustion takes place. The heat of the carbon evaporates the oxygen in its immediate vicinity, and the carbon burns with great brilliancy and violence, forming carbonic acid, which is largely frozen in the liquid before it reaches the surface, and falls back to the bottom of the dish, so that the combustion is maintained and its products retained within the dish. A beefsteak may be frozen in liquid air to such brittleness that it is shattered like a china plate when struck a slight blow. The intense cold of liquid air does not destroy the vitality or germinating power of seed, but produces serious so-called burns on the flesh that destroy the tissues and do not heal for many months, and yet for a moment the finger may be dipped in liquid air with impunity because of the gaseous envelope with which the finger is temporarily surrounded.