By polishing or silvering the walls, the transference of heat by radiation is also reduced, for the polished surface both emits and absorbs radiation less rapidly than a dull surface, so that by polishing both surfaces the outer surface sends less radiant heat to the inner sur face, and the inner surface re flects back part of the heat re ceived by the outer surface. For this reason the vacuum vessel, with the space between the walls highly evacuated, and the walls silvered, is the most efficient apparatus for storing cold liquids, or for maintaining constant low temperatures. The vessels in general use in scientific laboratories are generally of glass, and sometimes of silica; those used in industry are commonly of metal. This space must be ex hausted, preferably by means of a molecular pump, and heated in an oven during the process to about 300° C, so as to remove absorbed moisture and gases from the glass. The heat transference into the vessel is dependent on three phenomena :—( I) radiation represented by when s is the surface area, a the emissivity of the surface, which is unity for dull black material, and a small fraction for silver, and and T2 are the absolute temperatures of the two surfaces; (2) conduction stated by Dewar and Briggs to be represented by sb(7Y— (T1-1- T2) ; (3) loss by conduction down the material of the neck and con vection by the gas inside it. The actual loss from a well ex hausted spherical glass vacuum flask with silvered walls and of about one kilogram capacity is about 25o grammes of liquid oxygen per diem. The loss from a metal vessel of the same size would be nearly three times as much, but the loss from a com mercial metal vessel of 3o kg. capacity might be as low as 4.8% per diem.
Vacuum vessels for scientific use are of a variety of shapes and sizes as indicated by the dia gram (fig. 17), in which a and b are in commonest form. A three walled vessel is shown in c, the intermediate wall reducing the heat transference by radiation. A vessel from which liquid can be drawn off at the bottom is shown in d. A convenient type of commercial copper vessel is shown in fig. 18. Its capacity might be as much as 3o kg. of liquid oxygen. It has an inner wall to reduce heat loss by radiation, and a pad of char coal enclosed between the metal inner wall and a piece of gauze on the under side helps to maintain the vacuum by absorbing traces of gas from the evacuated space.
Physical Measurements at Low Temperatures.—The solution of problems relating to the liquefaction of gases de mands the determination of their physical properties at low temperatures, and particularly the following: (I) boiling point and vapour pressures, (a) densities of liquid and vapour, (3) critical constants, (4) compressibilities, (5) specific heats. These investigations demand in the first place the means of maintaining sufficiently large spaces at constant temperature. This may be done conveniently by liquefying pure gases and allowing the liquid to evaporate in vacuum vessels under constant pressure. In this way the following ranges of temperatures may be maintained:— Methods of investigation at constant low temperatures have been very fully developed in the Leyden Cryogenic laboratory under Kammerlingh Onnes. Cryostats are in use there in which a constant low temperature can be maintained by generating the refrigerating medium actually in the apparatus at a sufficient rate to supply the loss by evaporation. For moderately low tem peratures the cryostats are of metal suitably lagged, with glass windows for the observation of the enclosed apparatus, but when very low temperatures are required the cold space must always be enclosed in a vacuum vessel. In the chemical or physical laboratory liquid air finds innumerable applications, such as the condensation of gases either for the purpose of separating the constituents of a mixture for analytical purposes, or for the preparation of pure substances by rectifying the liquid. (See CHEMISTRY ; Properties of Mixtures.) Liquid air also finds wide application for the production of high vacua, by introducing into the system to be evacuated a tube containing a suitable form of charcoal, cooled externally with liquid air.