LIQUEFACTION OF GASES (Lat. liquefae tio, from lig nefaccre. to make liquid. from liq (his. liquid, from iiquere. to be fluid 4- re, to make). Matter exists in different forms as solids, liquids, and gases, which may be roughly discriminated: A solid is matter in such a form that it keeps hoth its size and shape under all ordinary conditions; a liquid has as a rule a definite volume. but will assume the shape of any vessel in which it is placed: while, if allowed to fall freely so as to form drops, these are spheri cal: a gas, on the other hand. will take both the shape and volume of the solid which eon tains it. Water in these three forms is ice, ordinary water, and steam. Every one is famil iar with the fact that it is easy to make any one of these forms of water pass into any other. Thus, in nature. we have the formation of i hail. and snow, all of which in turn melt . nil form water or else vaporize and form steam. Similarly, we have the wat.r evaporating from the surfaces of lakes and oceans. and the rev. rse process. the formation of rain and dew. These changes of matter from ono form into another are called *ehanges of state.' and sub. t "Liquefaction of deals with the condi tion• under which matter in tl e ndi tion can be made to assun e the liquid one. The real test of matter being in the liquid state is its capability of fnrming drops, or of having a sharp surface of •epsration from 110 above The first philosopher who eonside•ed this prob lem was probably V:111 11011Dn111. who lived in the latter part of the sixteenth and the part of the seventeenth century. lle introdueed into science the word ...as and distimmishod be tween what he called 'gases' and 'vapors.' say ing that the latter could be condensed into the liquid state. while the former could not. This dis tinction has persisted in science up to within recent years. As early as the first year of the nineteenth century Dalton, the great English chemist, made the statement: "There can scarce ly be a doubt entertained respecting the reduci bility of all elastic fluids of whatever kind into liquids; and we ought not to despair of effecting it in low temperatures, and by strong pressures exerted by the unmixed gases." But up to the time of Faraday comparatively little experi mental work had been done.
In 1823, at the sug gestion of Sir Hum idify Davy, Faraday began a series of ex periments which ended in his securing n gen eral method for the liquefaction of gases which he applied to chlorine, sulphurous acid, carbonic aeid. cyanogen, and many others. This method was to generate the gases in large amounts in a small space so that the pressure was great and then to eool the gases, while under this high pressure, to a sufficiently low de gree. In 1845 Fara
day published a sec ond series of researches describing his observations on gases which he had suceeeded in liquefy ing. by making use of temperatures much lower than those which he had been able to ob tain in his first investigation. He thus sue ceeded in liquefying all gases then known. with the exception of hydrogen, oxygen, nitrogen, nitric oxide, and marsh gas. He subjected oxy gen to a pressure of about 1000 pounds to the square inch, hut it showed no signs of lique faction. In speaking. however, of his efforts to liquefy these so-called 'permanent gases,' he states: diminution of temperature and improved apparatus for pressure may very well be expected In give us these bodies in the liquid or solid state." This hope of Faraday's was rendered much more certain by the work of Andrews on the properties of carbon dioxide under tem peratures and pressures which began in 1863, and was published finally in 1869. The chief re sult of Andrews's work was the proof that for earbon dioxide, and probably therefore for other gases. there was a certain temperature. different for different substances, below which the gas must he cooled before any amount of pressure, however great, would muse it to liquefy. This temperature is called the 'critical temperature.' It was evident, them that the explanation of all previous work on the liquefaetion of gases which had been successful was the fact that the experi menters had used gases whose critical tempera tures were not lower than the lowest temperatures available in laboratories; while the lack of sue eess in the attempts to liquefy the permaneht gases was owing to the fact that the eritieal temperatures of these gases were extremely low.
The problem, then, to liquefy the permanent gases took the form of an investigation as to the means of securing extremely low temperatures.
.Nlethods for the production of low tempera tures may be conveniently divided into four classes.
Tut: I.:SE OF FREEZING-AIIXTURES. It has been known for many years that a mixture of common table salt and ice produced a temperature many degrees below that of melting ice when pure; namely. 0° C. The lowest temperature available by using this salt and ice, when mixed in suitable proportion, is —22° C. It was found, however, by Thilorier that by mixing solid carbon dioxide, which he was able to secure by a method to be described presently, and ordinary sulphuric ether the mixture had a temperature of —110' C. This mixture, always known as 'Thilorier's mixture,' was first studied by him in 18:34 and formed for many years the only means of seeuring extremely low temperatures. In fact, there is at the pres ent time no freezing-mixture in use that gives a temperature lower than this.