LIQUEFACTION OF GASES. 310 LIQUEFACTION OF GASES. ing that the lalltr could be condensed into the liquid state, while the former could not. This dis- tinction lias 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 eli'ecting 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- phry Davy, Faraday began a series of ex- periments which ended in his securing 4 gen- eral method for the liquefaction of gases which he applied to APPARATPH OP FAHADAY FOB chlorinC, SUlphurOUS THE LiQuKFACTiiis OF cHi.o- acid, carbonic acid. RISE, SULPBUHOU8 Aciu GAS, cvanogcn, and many ^^' V . ,,• V V others. This method The substances which bv , * ai their reaction produ.e the gas ^vas to generate the are placed in one end of a bent gases in large amounts glass tube and the open end j„ ^ gn,an space SO is then sealeil. If these reax-t^ ^i ^ xi Ing substances are in the end that the pressure was j). this is heated, thus causing great and then to cool theaetiveeyolutlonof theKas, tj, ,^^^,3 ^^hile und-r while theotherenil. /Mscooled 4,- i- , . f as far as necessar.v or possible, this nigli pressure, 10 Thus the gas in the end fc is a sufficiently low de- iinder high pressure and at a _ „ t_ jg45 p^ara- low temperature. o * ,,. , , day published a sec- ond series of researches describing his observations on gases which he had succeeded 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 suc- 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, but it showed no signs of lique- faction. In speaking, however, of his efforts to liquefy these so-called 'permanent gases,' he states: "Further diminution of temperature and improved apparatus for pressure may very well be expected to 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 varying 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 carbon dioxide, and probably therefore for other gases, there was a certain temperature. diflTerent for different substances, below which the gas must be cooled before any amount of pressure, however jrreat, would cause it to liquefy. This temperature is called the 'critical temperature.' It was evident, then, that the explanation of all previous work on the liquefaction of gases which had been successful was the fact that the experi- menters had used gases whose critical tempen- tures were not lower than the lowest temperatures available in laboratories; while the lack of suc- cess in the attempts to liquefy the permaneVit gases was owing to the fact that the critical temperatures of these gases were extremely low. The problem, tlien, to liquefy liic permanent gases took the form of an investigation as to the means of securing extremely low temperatures. Methods for the production of low tempera- tures may be conveniently divided into four classes. The Use of Freezing-Mixtures. 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 wlien pure ; namely, 0° C. The lowest temperature available bj- using this salt and ice, when mi.xed in suitable jiroportion, is — 22° C. It was found, however, by Thilorier that bj' 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. Tliis mixture, always known as 'Thilorier's mixture,' was first studied by hini in lS.'i4 and formed for many years the only means of securing extremely low temperatures. In fact, there is at the pres- ent time no freezing-mixture in use that gives a temperature lower than this. The Method of Ev.^poration. It was recog- nized by all the investigators in the subject of heat, notably by Leslie and Black, that when a liquid evaporated rapidly the surrounding ob- jects, and especially the solid vessels which con- tained the liquid, gave up a great deal of heat and that thus their temperature fell. This is owing to the fact that it requires heat to pro- duce evaporation : ami so. if evaporation ever takes place, a definite amount of heat must be taken away from the neighboring matter. In this manner Leslie and WoUaston showed that water could be frozen by means of its own evaporation. The first use. however, of this fact concerning the low temperatures produced by evaporation made to secure liquefaction of gases was in the investigation of Bussy in 1824. who observed that if liquid sulphurous acid was made to evaporate rapidly he could get a tem- jierature as low as — 0.1° C. and using this meth- od he liquefied chlorine, ammonia, and cyanogen, and. in fact, solidified the last-named gas. Tliis method of securing low temperature immediately came into general use and is still used in many processes. Method of Expansion. It was noted by many investigators early in the nineteenth century, if not before, that when a gas under high pres- sure is allowed to expand into the open air. or to escape through a small opening, it experiences a drop in temperature: and the laws concerning this fall were studied by Gay-Lussac and many others. The first practical use. however, of the method with reference to liquefying gases was made in 1834. while Thilorier liquefied and even solidified carbonic-acid gas by allowing it to ex- pand suddenly. He made use further of the low temperatures thus obtained by expansion in study- ing many physical properties of bodies. This method was extended also by Mitchell in 1830 and was used practically by Dr. .Tobn Gnrrie. who, in 1849. constructed an ice-machine based on the low temperature produced by the expan- sion of compressed air. This method of first compressing a gas, causing it to be cooled to .a temperature as extreme as possible by ordinary means, and then allowing it to expand sud- denly is the standard method now in use in all machines made for liquefying gases. There are two processes involved in this production of low
