LIQUEFACTION OF GASES. 311 LIQUEFACTION OF GASES. temperature by expansion. It is evident that, if a gas compressed in a cylinder is allowed to escape through a small opening, work will be done both in overcoming the pressure of the air and also in giving kinetic energy to the rapidly escaping gas. This work must be done by the gas itself which stays behind and, as it were, pushes out the other gas; and therefore its tem- perature falls. There is a second reason, how- ever, depending upon the fact first discovered in the experiments by .Joule and Thomson (now Lord Kelvin ) . This is that there is a change in the intrinsic energy of a gas when it occupies a small volume and its molecules are thus close together, and when it fills a larger volume and its molecules are far apart. It is observed that if oxygen or nitrogen or any of the ordinary gases, with the exception of hydrogen, is allowed to expand, doing no external work, the tempera- ture of the gas falls. In the case of hydro- gen, it was found that at ordinary temperatures if "free' expansion takes place, there being no e.xternal work done, the temperature rises. How- ever, if the initial temperature of the hydrogen is sufficiently low. then the expansion will pro- duce a further fall in temperature. Thus, when anj- gas is under pressure at a low temperature and is allowed to expand by escaping into a space at a lower pressure, this temperature will fall ; and, if it is under sufficiently high pressure, the gas may thus be liquefied. The Regenerative JIethod. In this method gas under pressure is allowed to expand, and then, having thus been cooled, it is driven back and made to circulate round the outside of the vessel which contains the compressed gas. In this manner the temperature of the compressed gas is being brought down continually lower and lower, until, finally, it is at a temperature so low that any further decrease in temperature will produce liquefaction. This method was first used by Siemens in 18.57. The first suggestion that this principle should be used in the scientific study of the liquefaction of air and other gases was due to Edwin .1. Houston in 1874. Since this time the regenerative principle has been used by several investigators, notably by Prof. Kamerlingh Onnes in 1804, and by Professor Dewar in 1895. In all the commercial forms of machines this principle forms one of the essential features of the apparatus. After the publication of Andrews's researches in 1869, work proceeded very rapidly, and finally, in December, 1877, oxygen was liquefied by two investigators working independently: Cailletet, one of the greatest ironmasters of France, ex- perimenting at his works at Chatillon-sur-Seine. and Pictet, a maker of machines for manufac- turing ice, working at Geneva. The latter lique- fied oxygen at a pressure of 320 atmospheres and a temperature of — 140° C. producing his low temperature by a combination of the methods of evaporation and expansion ; and the former com- pressed his oxygen and carbon monoxide to a pressure of 200 atmospheres, cooled it to a tem- perature of about — 28° C, and then allowed it to expand suddenly, Cailletet succeeded in his experiments on December 2d. and Pictet succeeded in his December 22d; an announcement was made to the world of the results of both at the meeting of the French .cademy on December 24. 1877. Neither of these obser^'ers obtained oxygen in any large amounts or in a static condition, prac- tically only a mist being formed. Cailletet pro- ceeded to liquefy both nitrogen and air; but both he and Pictet secured only the fainte-st evidence of the liquefaction of hydrogen. The first to obtain oxygen, nitrogen, and carbon monoxide in fairly large amounts were Wroblewski and Ol- szewski, who began their work at Cracow in 188:i. The method used by them was simply to produc-e a low temperature by a combination" of freezing- mixtures and the evaporation of liquids, and under these conditions if the pres.sure of the gas is sufficiently great it is liquefied. Experi- ments were made on hydrogen, but their greatest success was simply to obtain a slight froth and other signs of ebullition ; they failed to secure any drops of the liquid. These observers were the first to make a careful study of the physical properties of gases at low temperatures, meas- uring their critical temperatures and pressures, their boiling-points, isothermals, etc. Olszewski succeeded in .solidifying a nunil«'r of gases, espe- ciallj' carbon monoxide and nitrogen, and meas- ured their freezing-points. He was the first to make use of the evaporation of liquid o.vygen and liquid air as a means of producing low temperatures. APPARATUS OF DEWAR FOR THE LIQUEFACTION OF HVDRUUEN. a rppresents one of the c.vliiiden* containlDg b.vdrogien under the pressure of about 200 atmospheres* ; t i« a De- war flask coiitaiiiiiig liquid rarbonic acid evaporating under exhaustion; c is a similar tla^k containing liquid air; d is the regenerative coil through which the gas under pressure is pa*48ed. and over which, after bein^ cooleil by expansion, it is drawn : f; is the pin-hole nozzle through" which the compressed gaj3 expands, escaping at the rat4? of about 15 cubic fe^'t per second : e is an oiM»ning through which the gas can be withdrawn and again ciim- pressed ; /"is the valve b.v which the size of the pin-lii>le nozzle /! is regulated. The flask containing the regenera- tive coil is a doublv silvered vacuum vessel, and is sur- rounded with a space kept below —200^ C. The liquid h.ydrogen. when formed, drops from this vacuum vessel into another which is doubl.v insulated by being sur- rounded with a third vacuum vessel, The greatest advances made in recent years in the field of low-temperature research are un- doubtedly those due to the investigations of Pro- fessor Dewar at the Royal Institution, London, who has used in recent years the regenerative method in producing the requisite low tempera- tures. By this method he succeeded in obtain- ing liquid oxygen, nitrogen, and air in prac-
