from the name of the inventor, the Dewar flask, which is open at the top, but otherwise insulated from the temperature of the surrounding air by having a double wall, with a vacuum between the walls. The familiar thermos bottle is constructed on the same principle. In such a vessel liquid air can be kept for hours and even days, and it is thus available for use in many interesting laboratory experiments.

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.