: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 1839 and was used practically by Dr. John Currie, 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 male for liquefying gases. There are two processes involved in this production of low temperature by expansion. it is evident that, if a gas compressed in a cylinder is allowed to escape through a small opening, work will he 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 external 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 any 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.