could thus be studied to advantage, | molecular rush together of these two and the inquiry led to the acquisition of eagerly uniting elements. The phosmuch valuable information regarding phorus remained no less indifferent modes of procedure at low tempera- than cold steel is to the contact of cold tures.1 water. These have of late in the laboratory The critical temperature of oxygen is of the Royal Institution, been carried-171° (-113° C.). Above that point to an extraordinary degree of perfec- it remains obstinately gaseous; intertion. The experiments conducted there stitial movements are too active to are not only on a totally unprecedented permit its molecules to lay hold one of scale, but give evidence of remarkable another. Nitrogen is still less amesagacity in the adaptation of means to nable, its corresponding stage being an end. Professor Dewar has been reached only at the deeper depth of occupied with them during at least ten -233° (-147° C.). Such cold is unatyears, but their striking results were tainable by direct means; the intervenfirst made known to the general public tion of liquid oxygen is required to on the occasion of the Faraday cente- produce it. The principle of its emnary in 1891. The numerous audience ployment is easily explained. Mouncollected in the theatre of the Royal taineers know by experience that no Institution on Friday evening, June good tea is to be had at high altitudes, 26 of that year, were amazed to see because water cannot be sufficiently liquid oxygen freely on tap and drawn heated to infuse it properly. On the off, to a vulgar apprehension, smoking summit of Monte Rosa, the boiling hot. In point of fact it was boiling at point is reduced from 212° at sea-level a temperature of -296° F. (328° of to 185°. The change is simply an frost), its steaming appearance being effect of diminished atmospheric pressdue to the conversion of the moisture ure, and can accordingly be brought in the surrounding air into ice particles about by a few strokes of an air pump through contact with the swiftly escap- with less exertion than by an ascent of ing gas. When cleared, by filtering fifteen thousand feet. Now, boiling through blotting paper, of some fine oxygen, like boiling water, is colder in dust of solid carbonic acid, it wore the vacuo than in the open airso much aspect of limpid light blue water. A colder, indeed, that a temperature of few drops of it thrown, however, on a-328° is afforded by it, and this suffices genuine water surface fizzed and sput- to liquefy, not only nitrogen, but also tered like red-hot iron plunged into a atmospheric air. On June 2, 1892, the cool stream; and presently each was singular spectacle was witnessed durseen floating about in a self-made little ing one of Professor Dewar's lectures, cup of ice. Some alcohol poured into of common air, more than ordinarily the mysterious fluid became promptly warm and elastic owing to the heated a solid block. Yet alcohol resists the state of the room, trickling spontasternest Arctic rigors, freezing, indeed, neously into an open vessel surroundonly at -202° (-130° C.). Removed ing a test tube in which oxygen was from the oxygen, it thawed into a boiling under exhaustion. Air, we viscid substance, which could not be need scarcely say, is a mechanical mixinduced to burn until it had taken up ture of four parts of nitrogen with one heat enough to restore it to its normal of oxygen; and the latter, being the consistence. The chemical inertness less refractory to cold of the two ingreproduced by extreme cold was further dients, might have been expected to illustrated by the immersion of a piece condense first. Yet things happen of phosphorus in liquid oxygen. otherwise. The two gases liquefy tovivid outburst of light was visible, such gether, but evaporate separately. The as marks, at ordinary temperatures, the nitrogen boils off on its own account, leaving almost pure oxygen behind. Somewhat similarly, sea water freezes 1 Wiedemann's Annalen, Bde. xx. xxxi. xlii. No without depositing the salts dissolved | employed in pneumatic chemistry has in it, but abandons them in vaporizing. | been erected under Professor Dewar's The analogy is indeed far from being superintendence in the laboratory complete; yet it serves to show that where Davy and Faraday did their the behavior of the gases is not quite memorable work. It includes one gas, so anomalous as, at first sight, it appears. and two steam engines, four steel compressors, a couple of large air pumps, with wheelwork, shafts, and couplings quantum suff. The inner organic details of this machinery, however, conceal the real secret of its efficiency. They have been thought out and perfected by the application of uncommon inventive talent combined with pertinacious industry. Mental concentration has translated itself into a convergence Professor Dewar's crowning achievement, so far, has been the solidification both of air and nitrogen. A substance visually undistinguishable from ice results in each case. Unmixed oxygen refuses to freeze. Not, we may be sure, because it is incapable of assuming the solid state, but because the requisite conditions have still to be found out and produced. The incon- of ingenious devices upon a single gruity is, however, noteworthy between the facility, comparatively to nitrogen, with which it liquefies, and the difficulty with which it solidifies. object. The principle of the method adopted. for securing its realization is that of lowering boiling points by exhaustion. And now, we may well ask ourselves, Choice is made of a series of substances. how has all this been accomplished ? more and more difficult to liquefy, and For surely Professor Dewar cannot, each in turn is enlisted for the service like the pedestrian soul of Elpenor, of vanquishing the recalcitrance of its have crossed unsuccored the ocean fellows. Thus, by successive gradastream that lay between him and the tions, the startling temperature of "regions of thick-ribbed ice" to which-346° (−210° C.) has been reached. he has penetrated. What "white- The process (generally abridged in winged ship," then, has been at his practice) begins with the freezing of disposal? Which of the Olympians has carbonic acid through the rapid evapoaided him to distance his competitors ration of ether. Carbonic acid boils. in the race towards the goal of absolute under atmospheric pressure at -112o cold? The business was an arduous (-80° C.), but by compulsion of the one. Intellectual, moral, and material air pump at -166°. Nitrous oxide, at resources were needed for its prosecu- this stage of the descent, surrenders at tion, and needed without stint. In discretion, and, boiled in vacuo, affords point of scale alone, a prodigious ad- the means of liquefying ethylene, vance has been made. Only a few which, similarly treated, runs down the years ago a capillary tube held all the temperature to -229° F. Here at last oxygen that could be liquefied at one the almost irrepressible activity of oxytime; and our professor himself was gen particles is so much reduced that a content to experiment upon a mere pressure of fifteen hundred pounds to fairy thimbleful of a substance which, the square inch avails to bring them for rarity, might vie with "a hair from within the power of cohesion. The the great Cham's beard." Now it can accelerated evaporation of liquid oxyliterally be manufactured by the hogs-gen gives cold enough to liquefy air head. At a high cost, certainly. We and nitrogen; and these, again, worked much doubt whether— making allow-upon in double receivers by two potent ance for the outlay on the "plant" air pumps, combine their refrigerative any profit could be derived from selling it at 1001. a gallon; and a couple of gallons are often consumed in the researches of a single afternoon. The most powerful apparatus ever an forces to produce solid nitrogen experiment successfully performed for the first time in public on January 19, 1894. It seems for the present to mark the ne plus ultra of what can be done 3 by artificial cold. One further exploit | jacket," shows no signs of agitation, uum is produced in the outer shell of a double receiver, from which every trace of air has been expelled on the Torricellian plan. By then freezing The most indispensable of Pro- out the excess of mercury, an extraorfessor Dewar's cold-producing agents dinarily high degree of exhaustion can is ethylene, a specimen of which was be reached. The pressure of mercuexhibited by Faraday in 1845. It is rial vapor- -the only form of matter a compound of hydrogen and carbon, remaining in a bulb thus treated prepared by treating alcohol with amounts to no more than the four strong sulphuric acid. At the Royal hundred-thousandth part of a millionth Institution it is now manufactured of a millimetre. Yet the exterior apa hundredweight at a time, notwith-plication of a pad of cotton wool standing the "hairbreadth 'scapes" by steeped in liquid oxygen promptly conwhich the operation is attended unless denses this evanescent remnant of the most delicate precautions are ob- metallic gas into a bright mirror, just served. For it is highly explosive, as moisture deposits itself in ice on the and the slightest leakage of the pipes inside of a window pane during a night through which it is conducted may at of hard frost. Through the nearly any time lead to a dangerous accident. absolute void in the mirror-coated bulb None the less, its services are indis- an electric discharge can with the pensable, and are availed of to the greatest difficulty be made to pass; and utmost. Each fresh supply is made to the difficulty might not improbably do duty over and over again, by being amount to an impossibility could the put through an incessant round of sparsely strewn surviving molecules be alternate liquefaction and evaporation; swept away. Enclosed in such vacuous and nitrous oxide has a similar course spaces, liquid oxygen is virtually proof of cyclical duty imposed upon it. against the attacks of heat; and experiments upon its peculiarities can be conducted with ease and at leisure. Professor Dewar's experiments upon liquid oxygen were, to begin with, gravely hampered by the violence of Oxygen may be regarded as the its ebullition. No single particle of it leading terrestrial element. It forms remained a moment at rest. Deter-eight-ninths by weight of water, oneminations of its qualities were as fifth of the earth's atmosphere, and impracticable as would be delicate about one-half of its rocky crust. astronomical observations through the There is, however, no certainty that it undulating vapors close to the horizon. exists at depths exceeding, say, thirty But difficulties are often only a spur to miles. Oxidation is more likely to invention; and they here resulted in have been limited to the superficial the important contrivance of "vacuum layers of the cooling globe than to vessels" for preserving fluids at equa- have extended to its interior. More ble temperatures. Air molecules are significant to ourselves are the facts indefatigable carriers of heat. On that our bodies are mainly composed every square inch of cold surface ex- of this normally aërial substance; that posed to bombardment by them, they they are largely nourished and wholly strike in billions per second, and each, vitalized by it. Oxygen is in great as it retires, leaves behind some minute demand in the economy of nature. portion of its own thermal store. By Most other chemical elements crave hindering their access, however, the for union with it, and once they have process of temperature equalization | laid hold of it, they do not readily let it be virtually arrested. Hence, go. If ever present, then, in lunar air liquid oxygen, protected by a vacuum and oceans, it may very well have been can long ago "drunk up" by chemical changes its clear tinge to pronounced action. Extra-terrestrially, it occurs indigo, and there can be no question certainly in meteorites, and probably but that its formation as a gas in the in some of the planets; but neither atmosphere must tend to deepen the sun nor stars give any sign of being blue of the sky. This, however, is supplied with it. Dusky bands caused probably only a subsidiary effect. The by its absorption are, it is true, included fundamental explanation of the celesamong the hieroglyphics inscribed on tial azure is to be found―according the solar spectrum; but their strictly to the opinion of M. Olszewski, with telluric origin has been demonstrated which Professor Dewar cordially agrees by M. Janssen, who ascended Mont -in the true, native color of oxygen. Blanc for the second time last Septem- And its color inevitably results from ber with a view to studying their prog- the preponderance given to the blue ress towards extinction with elevation end of its spectrum by its stoppage of in the earth's atmosphere. The upshot many red, yellow, and orange rays. was to show that, outside its limits, no trace of them would be left. They are, then, of domestic production. The sun has nothing to do with them. The virtual immutability with change of state of the absorptive action upon light of gaseous oxygen corresponds with the persistence of its other qualities. After liquefaction it is as bad a conductor of heat and electricity as before, and has lost nothing of its thermal transparency. In one of Professor These atmospheric bands and rays are common to the spectra of aërial and liquid oxygen. The same prismatic elements, conspicuously absent from the light of the setting sun, are Dewar's striking experiments, a sphercut out of an electric beam which ical vessel filled with liquid oxygen has traversed a couple of inches of was made to act as a burning glass. liquid oxygen. This identity, partially The beams from an electric arc, brought shown by M. Olszewski in 1887,1 was to a focus by its refractive influence, completely ascertained by Professors actually ignited a piece of paper held Liveing and Dewar in 1889. It proves, there, while leaving the frigid lens not only that the molecules of oxygen traversed by them unaffected by so undergo no modification of structure in much as a momentary quickening of liquefying, but that a nine-hundredfold ebullition. Thus, radiant or ethereal condensation, combined with the re- heat encounters no resistance in passstraints of cohesion, leaves them free ing through oxygen, although molecto execute their characteristic vibra- ular heat, which can only creep along tions. This "persistence of molecular from particle to particle, finds the way constitution," Professor Dewar re-barred almost at the outset. marks, is the more noteworthy when it Again, gaseous oxygen was known to is considered that no compound of be magnetic; but it was very far from oxygen, so far as is known, gives the being anticipated that liquid oxygen absorptions of oxygen." 2 Not even would prove to be still more so. ozone, a compound of oxygen with it- December 10, 1891, Professor Dewar self, yet showing a radically different placed some of this substance in a spectrum. Liquid ozone is intensely rock-salt cup between and just below blue. Dissolved in liquid oxygen, it the poles of an electro-magnet. No sooner was the circuit completed than, to his ineffable surprise, the liquid sprang in one mass to the poles, and remained attached to them to the last A and B, which, in their opinion, "ought to throw drop left by its speedy evaporation. some light on the nature of the changes in passing Cotton wool moistened with oxygen from the gaseous state, as well as on the causes which produce the sequences of rays which are 1 Wiedemann's Annalen, Bd. xlii., p. 663. 2 Proceedings, Royal Society, vol. xlvi., p. 226. Renewed study, in 1892, of the spectrum of liquid oxygen disclosed to the same investigators alterations in the well-known bands called by Fraunhofer called channelled spectra." (Phil. Mag. August, 1892, p. 207.) On clung on with equal tenacity, until sucked dry by their overmastering attraction for its contents. The magnetic capability of liquid oxygen is, indeed, this they have invoked fire and frost, only one-thousandth that of iron; yet they have piled Ossa upon Pelion in its discovery is of far-reaching conse- the production of mechanical pressure, quence. they have come appreciably near to expelling the last molecule from otherwise vacuous spaces, they have invoked as allies the still obscure forces of electricity and magnetism, they have concentrated and analyzed light, they have dug deep into the solid earth, they have explored the abysses of the distant heavens, · Medias acies, mediosque per ignes Invenere viam. Liquid air appears to possess no distinctive qualities. It is magnetic just in proportion to the amount of oxygen entering into its composition; its spectrum is that of oxygen, weakened; its color is that of oxygen, attenuated. The part played by nitrogen is that of a simple diluent. Nitrogen is the most indifferent of substances. Its characteristics are mainly negative. It refuses to conduct heat or electricity, but it allows radiant heat and light to pass The general problem thus attempted without exacting any dues of absorp- to be solved is one which must fascinate tion. Chemically inert, it combines the curiosity of every thinking mind, even with oxygen only under the stress although it trenches—indeed, partly of electric excitement. Were it other-intrudes - upon the realm of the unwise, it could not discharge the func- knowable. What is the material basis tion of neutrality assigned to it in our of phenomena ? philosophers and physatmosphere. In every thunderstorm, icists alike ask themselves. Abstract it is true, a small quantity of nitric acid force from matter, and what remains ? is formed, which, carried to the ground Something that has mass, we can reply by rain, and absorbed by plants, helps without hesitation; something that reto supply the nitrogenous foodstuffs sists being set in motion when at rest, indispensable for the maintenance of and being brought to rest when in moanimal life. This fertilizing action of tion. But this something can have no electricity is of recent detection. It might be deemed superfluous, were vegetable organisms capable of appropriating nitrogen directly from the air; but, as a matter of fact, they derive from the soil their stores of this much needed material. weight, since gravity is a force; it must be devoid of consistence, because destitute of cohesion; it should be impalpable, since resistance to pressure implies activity, not mere passive impenetrability. To designate this caput mortuum "cosmic dust" does not go far towards explaining its real nature, which must, indeed, forever remain imperfectly known to us owing to our incapability of conceiving matter apart from force, or force apart from matter. The nearest way to the heart of the question, however, lies undoubtedly through the study of the relations of matter with heat. Matter, in a sense, lives by heat. All its transformations are effected, all its activities come into play, under thermal influence. What, then, would be the result of its total withdrawal ? The production of excessively low temperatures means much more than the performance of just a feat of arms in a scientific campaign. Liquid oxygen and solid atmospheric air are indeed trophies of a victory over nature; they represent the accomplishment of what, in the regular order of things, was especially designed to be impossible; but they represent much besides. For they are instruments of research as well as objects of curiosity. Under normal conditions at the surface of the earth, it is impossible to gain any thorough acquaintance with the essential There must be a lower limit to temcharacteristics of matter. The constant perature. For temperature measures aim of physicists has accordingly been molecular energy of motion; and since to widen the scope and vary the cir- this motion decreases regularly with cumstances of observation. To effect the progressive diminution of heat, we |