Professor Dewar discovered in the course of his researches that the best vessel for holding liquefied gases, in order to guard against access of heat from without, was a double-walled glass bulb, the space between the walls being com pletely exhausted of air, and the glass being silvered like a mirror so as to reflect radia tion. This vessel is called the 'Dewar bulb.' Important simplifica tions in the apparatus of Dewar for the lique faction of hydrogen have been made by Travers and others.
As soon as it was found possible to liquefy gases with comparative ease, several machines were made in order to obtain liquefied air for commercial purposes. The most important of these are those invented by Linde in Germany, llampson in England, and Tripler in America. These machines were all invented at about the same time (early in 1895), and all make use of the same principle—viz. the re generative method. With these machines liquid air can be obtained in almost quantities and in a comparatively short time.
Innumerable uses have been found for liquefied gases, both in scientific in vestigations and in indus trial applications. The low temperatures which can thus be obtained are useful in many chemical experi ments, and also in many spectroscopic investigations where it is essential to secure pure gases. Liquid carbonic acid is used in the preparation of all kinds of aerated waters, viz. 'soda water,' and also in the manufacture of salicylic acid. Enormous quantities of liquid sulphurous acid, which is used for many purposes, are now prepared for the market. Liquid nitrous oxide is used as an anesthetic for minor sur gical operations, especially in dentistry. Liquefied gases are used in the oper ation of most ice-machine plants. Liquid oxygen can be obtained commercially and is used extensively in hospitals. It has been thought that liquid air would be used as a motive power, but so far all attempts to control it have failed.
At temperatures as low as those of liquid air and solid hydrogen all the ordinary properties of matter would be expected to change, and a care ful study of these changes has been made by many investigators, notably by Professor Dewar. A few facts may be noted here. The electrical resistance of metals decreases as the temperature is lowered, and in many cases the relative order of metals with reference to their electrical re sistance is changed at low temperatures. Thus,
at ordinary temperatures silver is a better con found that bacteria can bear with impunity al most any temperature however low. Sterilization does not result even after an exposure tor one hour to a temperature of —182°; the germinat ing power of seeds is also unimpaired; blood, meat. and milk when sealed in glass tubes under go putrefaction in the ordinary course. In other words, life can exist at from the absolute zero, and probably 11111C11 under conditions which nevertheless almost completely stop chemi cal and molecular action.
It may he interesting to note in tabular form a few of the physical constants of the gases and their liquids.
du•tor than copper, whereas at —200° the re verse is true. The thermoelectric properties of bodies change to a marked degree. The mag netic moment of magnets is increased by 30 or 40 per cent. as the temperature is lowered fo —200°. The elastic constants of bodies increase by as much as four or five times when the temperature is lowered from + 15° to —182°. Rubber becomes brittle. Changes of color often take place, the original hue, however, returning in all cases when the temperature i restored. There are many curious phosphorescent phe nomena at low temperature. Milk becomes high ly phosphorescent. An egg shines as a globe of blue light. Chemical affinity is almost complete ly destroyed by cold : phosphorus. sodium, and potassium when placed in liquid oxygen remain absolutely unafTeeted. Photographic films re tain only about one-fifth of their ordinary sen sitiveness to light.
Some interesting experiments have been tried dealing with vital phenomena at low tempera tures. Warm-blooded animals, of course. perish at comparatively high temperatures. but it was In connection with these figures it should be stated that there are several lines of argument, based upon physical experiments, which lead scientists to believe that it is impossible for us to obtain by any physical means a temperature lower than about —273° C. This temperature is called 'absolute zero,' and the nearest approach made to it by any experiments so far perforated was in one by Professor Dewar when lie solidi fied hydrogen, obtaining a temperature only 15° above the absolute zero.