When it comes to the phenomena of light, we can, as is fitting, see our way a little more clearly, since, thanks to Thomas Young and his successors, we know pretty definitely what light really is. So when we learn that many substances change their color utterly at low temperatures—red things becoming yellow and yellow things white, for example—we can step easily and surely to at least a partial explanation. We know that the color of any object depends simply upon the particular ether waves of the spectrum which that particular substance absorbs; and it does not seem anomalous that molecules packed close together at—180° of temperature should treat the ether waves differently than when relatively wide apart at an ordinary temperature. Yet, after all, that may not be the clew to the explanation. The packing of the molecules may have nothing to do with it. The real explanation may lie in the change of the ether waves sent out by the vibrating molecule; indeed, the fact that the waves of radiant heat and those of light differ only in amplitude lends color to this latter supposition. So the explanation of the changed color of the cooled substance is at best a dubious one.
Another interesting light phenomenon is found in the observed fact that very many substances become markedly phosphorescent at low temperatures. Thus, according to Professor Dewar, "gelatine, celluloid, paraffine, ivory, horn, and india-rubber become distinctly luminous, with a bluish or greenish phosphorescence, after cooling to—180° and being stimulated by the electric light." The same thing is true, in varying degrees, of alcohol, nitric acid, glycerine, and of paper, leather, linen, tortoise-shell, and sponge. Pure water is but slightly luminous, whereas impure water glows brightly. On the other hand, alcohol loses its phosphorescence when a trace of iodine is added to it. In general, colored things are but little phosphorescent. Thus the white of egg is very brilliant but the yolk much less so. Milk is much brighter than water, and such objects as a white flower, a feather, and egg-shell glow brilliantly. The most remarkable substances of all, says Professor Dewar, whom I am all along quoting, are "the platinocyanides among inorganic compounds and the ketonic compounds among organic. Ammonium platinocyanide, cooled while stimulated by arc light, glows fully at—180°; but on warming it glows like a lamp. It seems clear," Professor Dewar adds, "that the substance at this low temperature must have acquired increased power of absorption, and it may be that at the same time the factor of molecular friction or damping may have diminished." The cautious terms in which this partial explanation is couched suggest how far we still are from a full understanding of the interesting phenomena of phosphorescence. That a molecule should be able to vibrate in such a way as to produce the short waves of light, dissevered from the usual linking with the vibrations represented by high temperature, is one of the standing puzzles of physics. And the demonstrated increase of this capacity at very low temperatures only adds to the mystery.
There are at least two of the low-temperature phenomena, however, that seem a little less puzzling—the facts, namely, that cohesion and rigidity of structure are increased when a substance is cooled and that chemical activity is very greatly reduced, in fact almost abolished. This is quite what one would expect a priori—though no wise man would dwell on his expectation in advance of the experiments—since the whole question of liquids and solids versus gases appears to be simply a contest between cohesive forces that are tending to draw the molecules together and the heat vibration which is tending to throw them apart. As a substance changes from gas to liquid, and from liquid to solid, contracting meantime, simply through the lessening of the heat vibrations of its molecules, we might naturally expect that the solid would become more and more tenacious in structure as its molecules came closer and closer together, and at the same time became less and less active, as happens when the solid is further cooled. And for once experiment justifies the expectation. Professor De-war found that the breaking stress of an iron wire is more than doubled when the wire is cooled to the temperature of liquid air, and all other metals are largely strengthened, though none other to quite the same degree. He found that a spiral spring of fusible metal, which at ordinary temperature was quickly drawn out into a straight wire by a weight of one ounce, would, when cooled to -182 deg, support a weight of two pounds, and would vibrate like a steel spring so long as it was cool. A bell of fusible metal has a distinct metallic ring at this low temperature; and balls of iron, tin, lead, or ivory cooled to -182 deg and dropped from a height, "in all cases have the rebound greatly increased. The flattened surface of the lead is only one-third what it would be at ordinary temperature." "These conditions are due solely to the cooling, and persist only while the low temperature lasts."
If this increased strength and hardness of a contracted metal are what one would expect on molecular principles, the decreased chemical activity at low temperatures is no less natural-seeming, when one reflects how generally chemical phenomena are facilitated by the application of heat. In point of fact, it has been found that at the temperature of liquid hydrogen practically all chemical activity is abolished, the unruly fluorine making the only exception. The explanation hinges on the fact that every atom, of any kind, has power to unite with only a limited number of other atoms. When the "affinities" of an atom are satisfied, no more atoms can enter into the union unless some atoms already there be displaced. Such displacement takes place constantly, under ordinary conditions of temperature, because the vibrating atoms tend to throw themselves apart, and other atoms may spring in to take the places just vacated—such interchange, in fact, constituting the essence of chemical activity. But when the temperature is reduced the heat-vibration becomes insufficient to throw the atoms apart, hence any unions they chance to have made are permanent, so long as the low temperature is maintained. Thus it is that substances which attack one another eagerly at ordinary temperatures will lie side by side, utterly inert, at the temperature of liquid air.
Under certain conditions, however, most interesting chemical experiments have been made in which the liquefied gases, particularly oxygen, are utilized. Thus Olzewski found that a bit of wood lighted and thrust into liquid oxygen burns as it would in gaseous oxygen, and a red-hot iron wire thrust into the liquid burns and spreads sparks of iron. But more novel still was Dewar's experiment of inserting a small jet of ignited hydrogen into the vessel of liquid oxygen; for the jet continued to burn, forming water, of course, which was carried away as snow. The idea of a gas-jet burning within a liquid, and having snow for smoke, is not the least anomalous of the many strange conceptions that the low-temperature work has made familiar.
PRACTICAL RESULTS AND ANTICIPATIONS
Such are some of the strictly scientific results of the low-temperature work. But there are other results of a more directly practical kind—neither more important nor more interesting on that account, to be sure, but more directly appealing to the generality of the non-scientific public. Of these applications, the most patent and the first to be made available was the one forecast by Davy from the very first—namely, the use of liquefied gases in the refrigeration of foods. Long before the more resistant gases had been liquefied, the more manageable ones, such as ammonia and sulphurous acid, had been utilized on a commercial scale for refrigerating purposes. To-day every brewery and every large cold-storage warehouse is supplied with such a refrigerator plant, the temperature being thus regulated as is not otherwise practicable. Many large halls are cooled in a similar manner, and thus made comfortable in the summer. Ships carrying perishables have the safety of their cargoes insured by a refrigerator plant. In all large cities there are ice manufactories using the same method, and of late even relatively small establishments, hotels, and apartment houses have their ice-machine. It seems probable that before long all such buildings and many private dwellings will be provided with a cooling apparatus as regularly as they are now equipped with a heating apparatus.
The exact details of the various refrigerator machines of course vary, but all of them utilize the principles that the laboratory workers first established. Indeed, the entire refrigerator industry, now assuming significant proportions, may be said to be a direct outgrowth of that technical work which Davy and Faraday inaugurated and prosecuted at the Royal Institution—a result which would have been most gratifying to the founder of the institution could he have forecast it. The usual means of distributing the cooling fluids in the commercial plants is by the familiar iron pipes, not dissimilar in appearance (when not in operation) to the familiar gas, water, and steam pipes. When operating, however, the pipes themselves are soon hidden from view by the thick coating of frost which forms over them. In a moist beer-cellar this coating is often several inches in thickness, giving a very characteristic and unmistakable appearance.
Another commercial use to which refrigerator machines are now put is in the manufacture of various drugs, where absolute purity is desirable. As different substances congeal at different temperatures, but the same substances at uniform pressure always at the same temperature, a means is afforded of freeing a drug from impurities by freezing, where sometimes the same result cannot be accomplished with like thoroughness by any other practicable means. Indeed, by this means impurities have been detected where not previously suspected. And Professor Ramsay has detected some new elementary substances even, as constituents of the air, which had previously not been dissociated from the nitrogen with which they are usually mixed.
Such applications of the refrigerator principles as these, however, though of vast commercial importance, are held by many enthusiasts to be but a bagatelle compared with other uses to which liquefied gases may some time be put. Their expectations are based upon the enormous potentialities that are demonstrably stored in even a tiny portion of, say, liquefied air. These are, indeed, truly appalling. Consider, for example, a portion of air at a temperature above its critical point, to which, as in Thilorier's experiments, a pressure of thirty-one tons to the square inch of the encompassing wall is being applied. Recall that action and reaction are equal, and it is apparent that the gas itself is pushing back—struggling against being compressed, if you will—with an equal power. Suppose the bulk of the gas is such that at this pressure it occupies a cubical space six inches on a side—something like the bulk of a child's toy balloon, let us say. Then the total outward pressure which that tiny bulk of gas exerts, in its desperate molecular struggle, is little less than five thousand tons. It would support an enormous building without budging a hair's-breadth. If the building weighed less than five thousand tons it would be lifted by the gas; if much less it would be thrown high into the air as the gas expanded. It gives one a new sense of the power of numbers to feel that infinitesimal atoms, merely by vibrating in unison, could accomplish such a result.