PROFESSOR DEWAR’S LECTURE-TABLE. FROM A PHOTOGRAPH TAKEN OF PROFESSOR DEWAR’S FIRST LECTURE ON THE LIQUEFACTION OF OXYGEN.
“Having obtained the liquid oxygen,” he continues, turning to the long table below the windows, “the question was how to store it for working purposes, with the least possible loss by evaporation. After various trials and experiments, we devised a set of vacuum vessels, each consisting of a tube or bottle for the liquid oxygen, sealed at the neck in a second tube or bottle, from which the air had been exhausted. I found the cheapest and best method of getting a vacuum to be the old Torricellian one of driving out the air with mercury vapor and then condensing the vapor. This had a further advantage. The tube containing the liquid oxygen was so cold that it froze the mercury vapor, and coated itself with a perfect metallic mirror, which by its reflection still further diminished the loss by radiated heat from the outside.” Without more ado, he lifted from a small frame one of the vacuum vessels referred to. It was a white glass jar, inside of which was what seemed to be a round metallic ball. Open at the neck, this ball was a bottle nearly filled with liquid oxygen, and by the light which reached it through the neck of the bottle it was a very clear pale blue liquid, which was evaporating quietly in a single thread of tiny bubbles, like a glass of champagne which has become nearly still.
It was one of those moments which Faraday would doubtless have regarded as solemn. To behold, for the first time, a liquid which your professors of chemistry have assured you was a gas and always would be a gas, is an experience which does not occur many times in a lifetime. After that, a sight of perpetual motion or the square of the circle would leave you calm. To know, furthermore, that this strange gas, which is the prime agent of all life, which is eight-ninths of all water and three-fourths of the entire earth, has been laid captive by science, reduced to a form which cannot fail to shed a flood of light on any number of abstruse problems in chemistry and mechanics, excites a deeper feeling. The pale blue liquid, which is strangely lustrous, seems truly magical. Moreover, it is a great surprise to see the liquid, which you expect to find under great pressure and ready to blow its containing vessel to pieces, evaporating quietly in the air, protected from heat by a vacuum on one side and its own cold vapor on the other. And so you can do nothing but stare at it in amazement, and gently shake the bottle, and turn from it to its discoverer with a feeling which is not entirely dissociated from awe. It has lost all its impressiveness to the professor, however, for he is busy preparing to illustrate some of its properties—an interesting introduction in themselves to the conditions which prevail twice as far below the freezing point of water as its boiling point lies above.
EARLY AND LATEST FORMS OF VESSELS FOR HOLDING LIQUEFIED OXYGEN.
He begins by pouring some of the oxygen into a test tube, white fumes appearing as he does so from the freezing of the moisture in the surrounding air. Then he drops into the liquid oxygen in the test tube a bit of phosphorus. Despite the flaming energy with which these two combine at ordinary temperatures, there is no action. The phosphorus is as unaffected as a chip of wood in water. He takes it out and pours in some pure alcohol, whose freezing point is much below that of mercury. It freezes with a slight sputter into what you can only call alcohol ice. He takes out the ice and holds a match to it. There is no sign of combustion. Placed on a glass dish the alcohol ice melts into a thick, oily liquid, which also declines to burn. In a few seconds, however, it warms to its ordinary thinness and burns as hungrily as ever. Then comes an exhibition of the “spheroidal state.” A drop of water thrown towards a red-hot stove does not touch the stove, because the evaporation is so rapid that the forming gas lifts the water and keeps it moving about. Precisely the same thing occurs when the oxygen is dropped over a flat glass dish at the temperature of the air, which is red-hot to the oxygen, comparatively speaking. It dances about, shaking and boiling furiously. As he pours it, a tiny drop splashes on the professor’s hand, and he flings it off with a quick jerk. “It makes a sore worse than a burn,” he explains, “if it ever touches the skin.” Then he drops some of it into water. It floats quietly, and as it boils off into gas, freezes a cup of water around it, floating about comfortably in its own boat. Then came curious evidence of its magnetic properties. Pouring a little into a flat cup of rock salt, he placed the cup between the poles of an electro-magnet, the one which Faraday used. The boiling liquid, the moment that the circuit was completed, flew to the two terminals en masse and clung there, still boiling away rapidly on the two points. A piece of cotton wool soaked in the liquid was held closely to one of the points, until all the oxygen had been sucked out of it, when it hung suspended between them. Liquid oxygen has a magnetic property, he said, which is about 1,000 as compared with 1,000,000, the magnetic property of iron. It is a non-conductor of electricity, and a spark one-tenth of a millimetre long from a coil machine, which would give a long spark in the air, would not pass through the liquid. It gave a flash now and then as a bubble of oxygen vapor came between the terminals. Liquid oxygen is, in fact, a high insulator.
Liquid oxygen at atmospheric pressure boils at −184° C. (−229.2° F.). By evaporating it under a diminished pressure, he gets much higher degrees of cold, and these have enabled him to both liquefy and solidify nitrogen and air. The experiment illustrating this was not only interesting; it was difficult to believe. In a double vacuum vessel the centre of which was an open test tube, and the second compartment a reservoir of liquid oxygen connected with an exhaust pump, he so lowered the pressure that the oxygen boiled tumultuously. As it did so, drops of clear liquid began to form on the sides of the test tube and gather at the bottom. It was liquid air, the oxygen and nitrogen of the atmosphere liquefying together at a temperature of −197.2° (−322.9° F.). He poured some of the liquid air into a second tube, and then showed how the nitrogen, which liquefies at a temperature fourteen degrees below oxygen, boiled off first. A smouldering splinter of wood held at the mouth of the tube was extinguished. A few moments later when it was again held there, it burst into brilliant flame. The nitrogen had all evaporated, and the oxygen was coming off. He explained that air became solid under pressure at −207° C. (−340.6° F.). It was a structureless glass, and he had not determined whether or not the oxygen in it was solid or was held suspended as a jelly. Nitrogen solidified under pressure at −210° C. (−346° F.). It was a white crystalline substance. He had no knowledge as yet whether oxygen crystallized in solidifying, but his belief was that it would not.
THE “COMPRESSORS.”
Concerning hydrogen, most elusive of all the gases, he had no present expectation of attaining liquefaction. Its critical point was below −210° C., and its boiling point −250° C. He had no means as yet of attacking the problem. In fact, the only thermometer he was able to use at these low temperatures was one which used hydrogen expansion as a measure of temperature. His main reliance in measuring low temperatures was a thermo-electric junction. Deeply interesting also was his description of liquid ozone, that strange form of oxygen which though identical with it in constitution is different in molecular arrangement. He obtains twenty per cent. of ozone from liquid oxygen by electrical stimulation, the ozone being of a very dark blue color, as dark as concentrated indigo. It is highly unstable, a beam of light having caused it to explode on one occasion, and its study even in small quantities requires all the delicacy of manipulation which is one of the special directions in which Professor Dewar as a chemist occupies the foremost rank.