and (1) the refractive power; (2), the power of rotating a ray of polarized light; and (3), the absorption spectra of both inorganic and organic bodies.
The meeting of the French Academy of Sciences, held the day before Christmas, 1877, was rendered memorable by the announcement that oxygen gas had been liquefied by two independent experimenters. Previous to that date, hydrogen, oxygen, nitrogen, nitric oxide, marsh-gas, and carbon-monoxide had resisted all attempts to liquefy them, whether in the hands of the skillful Faraday, the ingenious batterer, or the learned Andrews. Physicists and chemists, while admitting the class of so-called permanent gases, had for many years looked forward to their eventual liquefaction, yet the final success came as a surprise. This success was the result of the enterprise and ingenuity of a French iron-master, M. Cailletet, and of a Genevan manufacturer of ice-machines, Raoul Pictet, working independently. In each case, the process consisted in simultaneously exposing the gases to a very high pressure and a very low temperature. Pictet obtained the necessary pressure by generating the oxygen in a wrought-iron vessel strong enough to withstand an enormous strain, and the low temperature was secured by the rapid evaporation of liquid carbonic acid; Cailletet, whose apparatus was marked by extreme simplicity, obtained the great pressure by means of a hydraulic press, and the low temperature by suddenly diminishing the pressure upon the compressed gases. Descriptions of apparatus without diagrams are seldom intelligible; in this place they are superfluous, for we deal with results rather than with methods. Being ignorant of the "critical point" for oxygen, both experimenters employed a much greater pressure than necessary.
Since the initial successes, the problem of liquefying the quondam permanent gases has been successfully attacked by several experimenters, especially by Wroblewski and Olzewski, whose names indicate their nationality. By employing liquid ethylene (which boils in vacuo as low as -150° C. [-238° F.]) as a means of cooling the gases under pressure, both oxygen and nitrogen, as well as atmospheric air, have been liquefied at very moderate pressures.
Among the interesting results obtained are the following: at -102° C. (-152° F.), chlorine forms orange-colored crystals; at -115° C. (-175° F.), hydrochloric acid is a solid; at -118° C. (-180° F.), arsine forms white crystals; at -129° C. (-200° F.), ether solidifies; at -130° C. (-202° F.), absolute alcohol solidifies; at -184° C. (-299° F.), oxygen boils; at -191·2° C. (-312° F.), air boils; at -205° C. (-337° F.), air boils in vacuo. These extraordinary temperatures were measured by means of a hydrogen thermometer and by a thermopile. The lowest temperature measured (to date) is -225° C. (-373° F.), which was reached by reducing the pressure of solid nitrogen to 4 mm. mercury (Olzewski). Further noteworthy results are as follows: Nitrogen was obtained in "snow-like crys-
