Volume V Part 2 (1/2)

These six refractory gases now became a target for the experiments of a host of workers in all parts of the world. The resources of mechanical ingenuity of the time were exhausted in the effort to produce low temperatures on the one hand and high pressures on the other. Thus Andrews, in England, using the bath of solid carbonic acid and ether which Thilorier had discovered, and which produces a degree of cold of--80 Centigrade, applied a pressure of five hundred atmospheres, or nearly four tons to the square inch, without producing any change of state. Natterer increased this pressure to two thousand seven hundred atmospheres, or twenty-one tons to the square inch, with the same negative results. The result of Andrews' experiments in particular was the final proof of what Cagniard de la Tour had early suspected and Faraday had firmly believed, that pressure alone, regardless of temperature, is not sufficient to reduce a gas to the liquid state. In other words, the fact of a so-called ”critical temperature,” varying for different substances, above which a given substance is always a gas, regardless of pressure, was definitively discovered. It became clear, then, that before the resistant gases would be liquefied means of reaching extremely low temperatures must be discovered. And for this, what was needed was not so much new principles as elaborate and costly machinery for the application of a principle long familiar--the principle, namely, that an evaporating liquid reduces the temperature of its immediate surroundings, including its own substance.

Ingenious means of applying this principle, in connection with the means previously employed, were developed independently by Pictet in Geneva and Cailletet in Paris, and a little later by the Cracow professors Wroblewski and Olzewski, also working independently. Pictet, working on a commercial scale, employed a series of liquefied gases to gain lower and lower temperatures by successive stages. Evaporating sulphurous acid liquefied carbonic acid, and this in evaporating brought oxygen under pressure to near its liquefaction point; and, the pressure being suddenly released (a method employed in Faraday's earliest experiments), the rapid expansion of the compressed oxygen liquefies a portion of its substance. This result was obtained in 1877 by Pictet and Cailletet almost simultaneously. Cailletet had also liquefied the newly discovered acetylene gas. Five years later Wroblewski liquefied marsh gas, and the following year nitrogen; while carbonic oxide and nitrous oxide yielded to Olzewski in 1884. Thus forty years of effort had been required to conquer five of Faraday's refractory gases, and the sixth, hydrogen, still remains resistant. Hydrogen had, indeed, been seen to a.s.sume the form of visible vapor, but it had not been reduced to the so-called static state--that is, the droplets had not been collected in an appreciable quant.i.ty, as water is collected in a cup. Until this should be done, the final problem of the liquefaction of hydrogen could not be regarded as satisfactorily solved.

More than another decade was required to make this final step in the completion, of Faraday's work. And, oddly enough, yet very fittingly, it was reserved for Faraday's successor in the chair at the Royal Inst.i.tution to effect this culmination. Since 1884 Professor Dewar's work has made the Royal Inst.i.tution again the centre of low-temperature research. By means of improved machinery and of ingenious devices for s.h.i.+elding the substance operated on from the accession of heat, to which reference will be made more in detail presently, Professor Dewar was able to liquefy the gas fluorine, recently isolated by Moussan, and the recently discovered gas helium in 1897. And in May, 1898, he was able to announce that hydrogen also had yielded, and for the first time in the history of science that* elusive substance, hitherto ”permanently”

gaseous, was held as a tangible liquid in a cuplike receptacle; and this closing scene of the long struggle was enacted in the same laboratory in which Faraday performed the first liquefaction experiment with chlorine just three-quarters of a century before.

It must be noted, however, that this final stage in the liquefaction struggle was not effected through the use of the principle of evaporating liquids which has just been referred to, but by the application of a quite different principle and its elaboration into a perfectly novel method. This principle is the one established long ago by Joule and Thomson (Lord Kelvin), that compressed gases when allowed to expand freely are lowered in temperature. In this well-known principle the means was at hand greatly to simplify and improve the method of liquefaction of gases, only for a long time no one recognized the fact. Finally, however, the idea had occurred to two men almost simultaneously and quite independently. One of these was Professor Linde, the well-known German experimenter with refrigeration processes; the other, Dr. William Hampson, a young English physician. Each of these men conceived the idea--and ultimately elaborated it in practice--of acc.u.mulating the cooling effect of an expanding gas by allowing the expansion to take place through a small orifice into a chamber in which the coil containing the compressed gas was held. In Dr. Hampson's words:

”The method consists in directing all the gas immediately after its expansion over the coils which contain the compressed gas that is on its way to the expansion-point. The cold developed by expansion in the first expanded gas is thus communicated to the oncoming compressed gas, which consequently expands from, and therefore to, a lower temperature than the preceding portion. It communicates in the same way its own intensified cold to the succeeding portion of compressed gas, which, in its turn, is made colder, both before and after expansion, than any that had gone before. This intensification of cooling goes on until the expansion-temperature is far lower than it was at starting; and if the apparatus be well arranged the effect is so powerful that even the smaller amount of cooling due to the free expansion of gas through a throttle-valve, though p.r.o.nounced by Siemens and Coleman incapable of being utilized, may be made to liquefy air without using other refrigerants.”

So well is this principle carried out in Dr. Hamp-son's apparatus for liquefying air that compressed air pa.s.sing into the coil at ordinary temperature without other means of refrigeration begins to liquefy in about six minutes--a result that seems almost miraculous when it is understood that the essential mechanism by which this is brought about is contained in a cylinder only eighteen inches long and seven inches in diameter.

As has been said, it was by adopting this principle of self-intensive refrigeration that Professor Dewar was able to liquefy hydrogen. More recently the same result has been attained through use of the same principle by Professor Ramsay and Dr. Travers at University College, London, who are to be credited also with first publis.h.i.+ng a detailed account of the various stages of the process. It appears that the use of the self-intensification principle alone is not sufficient with hydrogen as it is with the less volatile gases, including air, for the reason that at all ordinary temperatures hydrogen does not cool in expanding, but actually becomes warmer. It is only after the compressed hydrogen has been cooled by immersion in refrigerating media of very low temperature that this gas becomes amenable to the law of cooling on expansion. In the apparatus used at University College the coil of compressed hydrogen is pa.s.sed successively through (1) a jar containing alcohol and solid carbonic acid at a temperature of--80 Centigrade; (2) a chamber containing liquid air at atmospheric pressure, and (3) liquid air boiling in a vacuum bringing the temperature to perhaps 2050 Centigrade before entering the Hampson coil, in which expansion and the self-intensive refrigeration lead to actual liquefaction. With this apparatus Dr. Travers succeeded in producing an abundant quant.i.ty of liquid hydrogen for use in the experiments on the new gases that were first discovered in the same laboratory through the experiments on liquid air--gases about which I shall have something more to say in another chapter.

PRINCIPLES AND EXPERIMENTS

At first blush it seems a very marvellous thing, this liquefaction of substances that under all ordinary conditions are gaseous. It is certainly a little startling to have a cup of clear, water-like liquid offered one, with the a.s.surance that it is nothing but air; still more so to have the same air presented in the form of a white ”avalanche snow.” In a certain sense it is marvellous, because the mechanical difficulties that have been overcome in reducing the air to these unusual conditions are great. Yet, in another and broader view, there is nothing more wonderful about liquid air than about liquid water, or liquid mercury, or liquid iron. Long before air was actually liquefied, it was perfectly understood by men of science that under certain conditions it could be liquefied just as surely as water, mercury, iron, and every other substance could be brought to a similar state. This being known, and the principles involved understood, had there been nothing more involved than the bare effort to realize these conditions all the recent low-temperature work would have been mere scientific child's-play, and liquid air would be but a toy of science. But in point of fact there are many other things than this involved; new principles were being searched for and found in the course of the application of the old ones; new light was being thrown into many dark corners; new fields of research, some of them as yet barely entered, were being thrown open to the investigator; new applications of energy, of vast importance not merely in pure science but in commercial life as well, were being made available. That is why the low-temperature work must be regarded as one of the most important scientific accomplishments of our century.

At the very outset it was this work in large measure which gave the final answer to the long-mooted question as to the nature of heat, demonstrating the correctness of Count Rumford's view that heat is only a condition not itself a substance. Since about the middle of the century this view, known as the mechanical theory of heat, has been the constant guide of the physicists in all their experiments, and any one who would understand the low-temperature phenomena must keep this conception of the nature of heat clearly and constantly in mind. To understand the theory, one must think of all matter as composed of minute isolated particles or molecules, which are always in motion--vibrating, if you will. He must mentally magnify and visualize these particles till he sees them quivering before him, like tuning-forks held in the hand. Remember, then, that, like the tuning-fork, each molecule would, if left to itself, quiver less and less violently, until it ran down altogether, but that the motion thus lessening is not really lost. It is sent out in the form of ether waves, which can set up like motion in any other particles which they reach, be they near or remote; or it is transmitted as a direct push--a kick, if you will--to any other particle with which the molecule comes in physical contact.

But note now, further, that our molecule, while incessantly giving out its energy of motion in ether waves and in direct pushes, is at the same time just as ceaslessly receiving motion from the ether waves made by other atoms, and by the return push of the molecules against which it pushes. In a word, then, every molecule of matter is at once a centre for the distribution of motion (sending out impulses which affect, sooner or later, every other atom of matter in the universe), and, from the other point of view, also a centre for the reception of motion from every direction and from every other particle of matter in the universe.

Whether any given molecule will on the whole gain motion or lose it depends clearly on the simple mechanical principles of give and take.

From equally familiar mechanical principles, it is clear that our vibrating molecule, in virtue of its vibrations, is elastic, tending to be thrown back from every other molecule with which it comes in contact, just as a vibrating tuning-fork kicks itself away from anything it touches. And of course the vigor of the recoil will depend upon the vigor of the vibration and the previous movements. But since these movements const.i.tute temperature, this is another way of saying that the higher the temperature of a body the more its molecules will tend to spring asunder, such separation in the aggregate const.i.tuting expansion of the ma.s.s as a whole. Thus the familiar fact of expansion of a body under increased temperature is explained.

But now, since all molecules are vibrating, and so tending to separate, it is clear that no unconfined ma.s.s of molecules would long remain in contiguity unless some counter influence tended to draw them together.

Such a counter influence in fact exists, and is termed the ”force” of cohesion. This force is a veritable gravitation influence, drawing every molecule towards every other molecule. Possibly it is identical with gravitation. It seems subject to some law of decreasing in power with the square of the distance; or, at any rate, it clearly becomes less potent as the distance through which it operates increases.

Now, between this force of cohesion which tends to draw the molecules together, and the heat vibrations which tend to throw the molecules farther asunder, there seems to be an incessant battle. If cohesion prevails, the molecules are held for the time into a relatively fixed system, which we term the solid state. If the two forces about balance each other, the molecules move among themselves more freely but maintain an average distance, and we term the condition the liquid state. But if the heat impulse preponderates, the molecules (unless restrained from without) fly farther and farther asunder, moving so actively that when they collide the recoil is too great to be checked by cohesion, and this condition we term the gaseous state.

Now after this statement, it is clear that what the low-temperature worker does when he would liquefy a gas is to become the champion of the force of cohesion. He cannot directly aid it, for so far as is known it is an unalterable quant.i.ty, like gravitation. But he can accomplish the same thing indirectly by weakening the power of the rival force. Thus, if he encloses a portion of gas in a cylinder and drives a piston down against it, he is virtually aiding cohesion by forcing the molecules closer together, so that the hold of cohesion, acting through a less distance, is stronger. What he accomplishes here is not all gain, however, for the bounding molecules, thus jammed together, come in collision with one another more and more frequently, and thus their average activity of vibration is increased and not diminished; in other words, the temperature of the gas has risen in virtue of the compression. Compression alone, then, will not avail to enable cohesion to win the battle.

But the physicist has another resource. He may place the cylinder of gas in a cold medium, so that the heat vibrations sent into it will be less vigorous than those it sends out. That is a blow the molecule cannot withstand. It is quite impotent to cease sending out the impulses however little comes in return; hence the aggregate motion becomes less and less active, until finally the molecule is moving so sluggishly that when it collides with its fellow cohesion is able to hold it there.

Cohesion, then, has won the battle, and the gas has become a liquid.

Such, stated in terms of the mechanical theory of heat, is what is brought to pa.s.s when a gas is liquefied in the laboratory of the physicist. It remains only to note that different chemical substances show the widest diversity as to the exact point of temperature at which this balance of the expansive and cohesive tendencies is affected, but that the point, under uniform conditions of pressure, is always the same for the same substance. This diversity has to do pretty clearly with the size of the individual molecules involved; but its exact explanation is not yet forthcoming, and, except in a general way, the physicist would not be able to predict the ”critical temperature” of any new gas presented to him. But once this has been determined by experiment, he always knows just what to expect of any given substance. He knows, for example, that in a mixture of gases hydrogen would still remain gaseous after all the others had a.s.sumed the liquid state, and most of them the solid state as well.

These mechanical conceptions well in mind, it is clear that what the would-be liquefier of gases has all along sought to attain is merely the insulation of the portion of matter with which he worked against the access of heat-impulse from its environment. It is clear that were any texture known which would permit a heat-impulse to pa.s.s through it in one direction only, nothing more would be necessary than to place a portion of gas in such a receptacle of this substance, so faced as to permit egress but not entrance of the heat, and the gas thus enclosed, were it hydrogen itself, would very soon become liquid and solid, through spontaneous giving off of its energy, without any manipulation whatever. Contrariwise, were the faces of the receptacle reversed, a piece of iron placed within it would be made red-hot and melted though the receptacle were kept packed in salt and ice and no heat applied except such as came from this freezing mixture. One could cook a beefsteak with a cake of ice had he but such a material as this with which to make his stove. Not even Rumford or our modern Edward Atkinson ever dreamed of such economy of fuel as that.

But, unfortunately, no such substance as this is known, nor, indeed, any substance that will fully prevent the pa.s.sage of heat-impulses in either direction. Hence one of the greatest tasks of the experimenters has been to find a receptacle that would insulate a cooled substance even partially from the incessant bombardment of heat-impulses from without.

It is obvious that unless such an insulating receptacle could be provided none of the more resistent gases, such as oxygen, could be long kept liquid, even when once brought to that condition, since an environment of requisite frigidity could not practicably be provided.

But now another phase of the problem presents itself to the experimenter. Oxygen has a.s.sumed the quiescent liquid state, to be sure, but in so doing it has fallen below the temperature of its cooling medium; hence it is now receiving from that medium more energy of vibration than it gives, and unless this is prevented very soon its particles will again have power to kick themselves apart and resume the gaseous state. Something, then, must be done to insulate the liquefied gas, else it will retain the liquid state for too short a time to be much experimented with. How might such insulation be accomplished?

The most successful attack upon this important problem has been made by Professor Dewar. He invented a receptacle for holding liquefied gases which, while not fulfilling the ideal conditions referred to above, yet accomplishes a very remarkable degree of heat insulation. In consists of a gla.s.s vessel with double walls, the s.p.a.ce between which is rendered a vacuum of the highest practicable degree. This vacuum, containing practically no particles of matter, cannot, of course, convey heat-impulses to or from the matter in the receptacle with any degree of rapidity. Thus one of the two possible means of heat transfer is shut off and a degree of insulation afforded the liquefied substance. But of course the other channel, ether radiation, remains. Even this may be blocked to a large extent, however, by leaving a trace of mercury vapor in the vacuum s.p.a.ce, which will be deposited as a fine mirror on the inner surface of the chamber. This mirror serves as an admirable reflector of the heat-rays that traverse the vacuum, sending more than half of them back again. So, by the combined action of vacuum and mirror, the amount of heat that can penetrate to the interior of the receptacle is reduced to about one-thirtieth of what would enter an ordinary vessel. In other words, a quant.i.ty of liquefied gas which would evaporate in one minute from an ordinary vessel will last half an hour in one of Professor Dewar's best vacuum vessels. Thus in one of these vessels a quant.i.ty of liquefied air, for example, can be kept for a considerable time in an atmosphere at ordinary temperature, and will only volatilize at the surface, like water under the same conditions, though of course more rapidly; whereas the same liquid in an ordinary vessel would boil briskly away, like water over a fire. Only, be it remembered, the air in ”boiling” is at a temperature of about one hundred and eighty degrees below zero, so that it would instantly freeze almost any substance placed into it. A portion of alcohol poured on its surface will be changed quickly into a globule of ice, which will rattle about the sides of the vessel like a marble. That is not what one ordinarily thinks of as a ”boiling” temperature.

If the vacuum vessel containing a liquefied gas be kept in a cold medium, and particularly if two vacuum tubes be placed together, so that no exposed surface of liquid remains, a portion of liquefied air, for example, may be kept almost indefinitely. Thus it becomes possible to utilize the liquefied gas for experimental investigation of the properties of matter at low temperatures that otherwise would be quite impracticable. Great numbers of such experiments have been performed in the past decade or so by all the workers with low temperatures already mentioned, and by various others, including, fittingly enough, the holder of the Rumford professors.h.i.+p of experimental physics at Harvard, Professor Trowbridge. The work of Professor Dewar has perhaps been the most comprehensive and varied, but the researches of Pictet, Wroblewski, and Olzewski have also been important, and it is not always possible to apportion credit for the various discoveries accurately, since the authorities themselves are in unfortunate disagreement in several questions of priority. But in any event, such questions of exact priority have no great interest for any one but the persons directly involved. We may quite disregard them here, confining attention to the results themselves, which are full of interest.

The questions investigated have to do with the physical properties, such as electrical conductivity, magnetic condition, light-absorption, cohesion, and chemical affinities of matter at excessively low temperatures. It is found that in all these regards most substances are profoundly modified when excessively cooled. Thus if a piece of any pure metal is placed in an electric circuit and plunged into liquid air, its resistance to the pa.s.sage of the electricity steadily decreases as the metal cools, until at the temperature of the liquid it is very trifling indeed. The conclusion seems to be justified that if the metal could be still further cooled until it reached the theoretical ”absolute zero,”

or absolutely heatless condition, the electrical resistance would also be nil. So it appears that the heat vibrations of the molecules of a pure metal interfere with the electrical current. The thought suggests itself that this may be because the ether waves set up by the vibrating molecules conflict with the ether strain which is regarded by some theorists as const.i.tuting the electrical ”current.” But this simple explanation falters before further experiments which show, paradoxically enough, that the electrical resistance of carbon exactly reverses what has just been said of pure metals, becoming greater and greater as the carbon is cooled. If an hypothesis were invented to cover this case there would still remain a puzzle in the fact that alloys of metals do not act at all like the pure metals themselves, the electrical resistance of such alloys being, for the most part, unaffected by changed temperature. On the whole, then, the facts of electrical conduction at low temperatures are quite beyond the reach of present explanation. They must await a fuller knowledge of molecular conditions in general than is at present available--a knowledge to which the low-temperature work itself seems one of the surest channels.