Liquid air looks much like water, except for its slight bluish color. It boils—i. e., changes back to ordinary air—at a temperature only slightly above that at which it is produced, and this boiling, of course, goes on rapidly at the surface of the liquid, owing to absorption of heat from the air above. Liquid air is lighter than water, upon which it consequently will float. A cubic foot of liquid air is the equivalent of about 800 cubic feet of ordinary air at 60° Fahrenheit and atmospheric pressure.
The curious effects of liquid air, only a few of which can be mentioned here, are not irrelevant to the subject of atmospheric resources, since they aid in various ways in carrying out important scientific researches. Almost all liquids are solidified and almost all solids are hardened and stiffened by immersion in liquid air. Alcohol is promptly frozen in it, and at the same time gives out so much heat that the liquid air boils violently and the congealing alcohol overflows the vessel in a little avalanche of snow. India rubber becomes as brittle as glass. Meats become so hard that when struck by a hammer they ring like steel. Chemical action is enormously reduced by exposure to the low temperature of liquid air, and so is the electric resistance of metals. One might suppose that such a temperature would be fatal to all forms of life, but this is not the case. A goldfish, frozen solid in liquid air, revives and swims vigorously a few seconds after being replaced in water. Bacteria survive hours of exposure to the temperature of liquid air, while the seeds of higher plants, even after several days of similar treatment, sprout the same as other seeds.
Most of the atmospheric gases have not only been liquefied, but also frozen solid. An important exception is helium, which has been liquefied only at a temperature of 452° below zero Fahrenheit. The remarkable feat of liquefying helium was accomplished in 1908 by the Dutch physicist Kamerlingh Onnes, who subsequently, in his attempts to solidify this substance, attained the unprecedented temperature of less than 2 (Centigrade) degrees above “absolute zero,” or 456° below zero Fahrenheit, by the rapid evaporation of the liquid under greatly reduced pressure.
Exploring the Upper Air. Left: Beginning of a pilot-balloon flight. Right: Sending up a sounding-balloon. Note the parachute, which wafts the basket of instruments gently to the ground after the balloon bursts. (Photographs from U. S. Weather Bureau.)
Although, when air is liquefied, the oxygen and nitrogen are condensed simultaneously, the latter has a lower boiling point than the former and therefore passes off more rapidly when the liquid is allowed to evaporate. This fact makes it possible to separate the two substances, by the process known as “fractional distillation,” and hence liquid air plants have been established for the special purpose of manufacturing oxygen and nitrogen, for both of which there is a large and growing commercial demand. Scores of millions of cubic feet of oxygen are used every year in the wonderfully efficient process of welding metals with the oxyacetylene blowpipe, the flame of which has a temperature of about 6,000° Fahrenheit. Most of the supply now comes from liquid air. An equally large amount is used in a recently introduced method of cutting metal. The object to be cut is first heated to incandescence, after which a jet of oxygen is played upon it. The metal actually burns away in the stream, and a clean cut is made like that of a saw. It is interesting to reflect, when we fill our lungs with oxygen in order to keep our bodily machinery in operation, that the same atmospheric gas is applied to the building of motor cars, bicycles, safes, boilers, and battleships. Cartridges made of lampblack, dipped for a few moments in liquid oxygen and then primed with a fulminate cap, constitute an explosive as powerful as dynamite and much cheaper to produce. A small percentage of oxygen added to the air supplied to blast furnaces has been found to effect a great saving of fuel used in the furnace.
Meteorograph for use with Sounding Balloon. (Fergusson pattern. U. S. Weather Bureau, 1919.) The aluminum case, surrounded by hoops of rattan to protect the apparatus when it falls to the ground at the end of the flight, contains a set of very light self-registering meteorological instruments. (Photograph from U. S. Weather Bureau.)
