THE REGENERATIVE 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 1857. The first suggestion that this principle should be 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 1894, 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 Childllon-sur-Seine. and Pictet, a maker of machines for manufae luring 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 cum 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 Academy all December 24. 1877. Neither of these observers 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 l'ictet secured only the faintest evidence of the liquefaction of hydrogen. The first to
obtain oxygen, nitrogen. and carbon monoxide in fairly large amounts were Wroblewski and 01 szewski, who began their work at Cracow in 1883. The method used by them was simply to produce a low temperature by a combination of freezing mixtures and the evaporation of liquids, and under these condition; if the pressure 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; 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 temperature; and pressures, their boiling-points, isotbermals. etc. t)lszew-ki succeeded in solidifying a number of gases, espe cially carbon monoxide and nitrogen, and meas ured their freezing-points. Ile was the first to make use of the evaporation of liquid oxygen and liquid air as a means of producing low temperatures.
The greatest advances made in recent year; in the field of low-temperature research are un doubtedly those due to the invest4raticns of Pr. fessor Dewar at the Royal m. London. who has used in recent years the regenerative method in the requisite lot% tempera tures. Ity this method he succeeded in obtain ing liquid oxygen. nitrogen, and air in prac tically unlimited amounts; he was able to solidi fy air as early as 1893; on May 10, 1898, he ob tained liquid hydrogen in fairly large amounts, and in 1899 he succeeded in solidifying both oxygen and hydrogen. In fact, he has succeeded in liquefying all known gases with the exception of helium, a gas whose existence on the earth has only recently been discovered. All attempts to liquefy this gas have so far failed, although temperatures as low as —258° C. have been used. There is no reason to doubt, however, that this gas also can be liquefied if suitable means are used.