LIQUEFACTION OF GASES. It has been long known that most solids can be trans formed into liquids by the application of heat, and that many liquids can also be transformed into vapor by a further addition of heat. Con versely, 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 existed between gases and vapors, vapors being condensible to the liquid form, while gases were believed to be permanently aeriform, and not condensible.
In the early part of the 19th century the validity of this distinction came to be doubted, and Faraday, at the suggestion of Davy, un dertook the systematic study of the question. He succeeded in reducing to the liquid form quite a number of gases that had previously resisted liquefaction. His general method con sisted in generating the gas in large quantities in a limited space, so as to produce a very high ,pressure, 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 V-shaped glass, tube, one of whose legs contains a chemical preparation suit able for the generation of the gas in question, while the other end dips into a freezing mix ture, the tube being hermetically sealed. If cyanide of mercury be heated in one of the legs of a tube of this kind, for example, cy anogen gas is generated in such quantities that the pressure causes a large part of it to con dense in the chilled end of the tube.
Chlorine was liquefied by Faraday in this manner in 1823. Shortly afterward Thilorier succeeded in liquefying and solidifying carbon dioxide by the combined of intense cold and great pressure; iard de la Tour, Regnault, Natterer and many other experi menters 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 re sisted all attempts at liquefaction, however, and these were still called °permanent gases,* although the conviction had forced itself upon physicists that all gases could be conquered if the necessary conditions could be discovered.
The subject was in this state when Andrews undertook his classical study of the phenomena of liquefaction of carbon dioxide. In 1863 he made the following announcement: °On par tially liquefying carbonic acid by pressure alone, and at the same time gradually raising the temperature to 31° C. the surface of demar cation .between the liquid and gas became fainter, lost its curvature, and at last disap peared. The space was then occupied by a homogeneous fluid which exhibited, when the pressure was suddenly diminished or the tem perature slightly lowered, a peculiar appearance of moving or flickering striae throughout its entire mass. At temperatures above 31° C. no apparent liquefaction of carbonic acid, or separation into two distinct 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 lique fied by any pressure whatever and this dis covery was soon verified in the cases of other gases. It was apparent, therefore, that for each gas there is a temperature above which it cannot be liquefied by any pressure. This tem perature is known as the °critical temperature,' and the pressure necessary to bring about lique faction at the critical temperature is called the °critical pressure.° The reason that oxygen, nitrogen and hydrogen had resisted previous attempts at liquefaction, even when the pressure was increased to 3,000 atmospheres, was that the critical temperatures of these gases are very low indeed — far below any temperature at which attempts at liquefaction had been made. The problem of liquefying the so-called °permanent gases° was then resolved into the production of exceedingly low temperatures.