GASES, Liquefaction of. It has also been long known that most solids can be transformed into liquids by the application of heat, and that many liquids can also be transformed into vapor by a further addition of heat. Conversely, it was known that certain aeriform substances, such as steam, can be converted into liquids by the mere abstraction of heat. It was believed, however, that an essential difference exists be tween gases and vapors, vapors being condensi ble to the liquid form, while gases were believed to be permanently aeriform, and not conden sible by any experimental means at our disposal. In the early part of the 19th century the validity of this distinction came to be doubted, and Faraday, at the suggestion of Davy, undertook the systematic study of the question. He suc ceeded in reducing to the liquid form quite a number of gases that had previously resisted liquefaction. His general method consisted in generating the gas in large quantities in a lim ited space, so as to produce a very high pres sure, under the influence of which (when the experiment was successful) the gas passed into the liquid state. The most convenient way of carrying out this experiment is to make use of an inverted U-shaped glass tube, one of whose legs contains a chemical preparation suitable for the generation of the gas in question, while the other end dips into a freezing mixture; the tube being hermetically sealed. If cyanide of mer cury be heated in one of the legs of a tube of this kind, for example, cyanogen gas is gener ated in such quantities that the pressure causes a large part of it to condense in the chilled end of the tube. Chlorine was liquefied by Faraday in this manner in 1823. Shortly afterward Thilorier succeeded in solidifying carbon dioxide by the combined application of intense cold and great pressure, and Cagniard de la Tour, Reg nault, Natterer, and many other experimenters. improved the methods in use with the result that many of the gases that had been previously regarded as non-condensible were reduced to the liquid form. Oxygen, hydrogen, nitrogen and some few other gases still resisted all at tempts at liquefaction, however, and these were still called °permanent gases? although the con viction had forced itself upon physicists that all gases could be conquered, if the necessary condi tions of success could be discovered. The sub ject was in this state when Andrews undertook his classical study of the phenomena of lique faction of carbon dioxide. In 1863 he made the following announcement : "On partially. lique fying carbonic acid by pressure alone, and grad ually raising at the same time the temperature to 88° F. (31° C.), the surface of demarcation
between the liquid and gas became fainter, lost its curvature and at last disappeared. The space was then occupied by a homogeneous fluid, which exhibited, when the pressure was suddenly diminished or the temperature slightly low ered, a peculiar appearance of moving or flicker ing striae throughout its entire mass. At tem peratures above 88° F. no apparent liquefaction of carbonic acid, or separation into two dis tinct forms of matter, could be effected, even when a pressure of 300 or 400 atmospheres was applied? It appeared, therefore, that a certain temperature exists, above which carbon dioxide cannot be liquefied by any pressure whatever; and this discovery was soon verified in the case of other gases. The temperature in question is known as the 'critical temperatures of the gas under experiment (For its numerical values in the cases of the more important gases, see CRITICAL Poncr). The reason that oxygen, ni trogen and hydrogen resisted previous attempts at liquefaction, even when the pressure was pushed to 3,000 atmospheres, was that the criti cal points of these gases are very low indeed,— far below any temperature at Which the attempt at liquefaction had been made. The problem of liquefying the so-called 'permanent gases" was, therefore, resolved into the production of ex ceedingly low temperatures. One means for the production of such temperatures was given by Thilorier, who showed that by mixing solid carbon dioxide with ordinary ether, a tempera ture as low as 165° F. below zero may be at tained. The cold produced by the expansion of the gases themselves has also been utilized for the production of the necessary degree of cold, and in the best modern forms of apparatus the gas, after being cooled by its own expansion, is furthermore caused to circulate about the pipes that are conducting fresh supplies of gas to the point at which the expansion takes place. In all cases, every care is taken to make use of any process or device which will lower the temperature of the gas; and by the strictest at tention to this general principle, it has been found possible to liquefy every known gas except possibly one or two of the rare gaseous elements recently discovered in the atmosphere. It is highly probable that these will also succumb, when they can be obtained in sufficient quantity to be treated by the same methods that have yielded success in the case of so obdurate a gas as hydrogen. Hy drogen was first liquefied, in quantity, by Dewar, in 1898. Consult Hardin, 'Rise and Devel opment of the Liquefaction of Gases)