PHYSICS. 457 



these refrigerants enable *be condensation to liquefy the oxygen, pro- 

 vided the pressure is above 50 atmospheres. But it is convenient to 

 have a stopcock attached to the tube, whereby a sudden expansion may 

 be efiected and a lower temperature thus obtained. By knowing the 

 volume of the liquid oxygen and the corresponding volume of the gas- 

 eous oxygen, the density of the liquid may be obtained. A rough ex- 

 periment gave Dewar 0.65 as the density near the critical point. As 

 early as 1883 Dewar had made experiments on liquefied marsh gas, and 

 pointed out the fact that the ratio of the critical temperature to the 

 critical pressure is proportional to the molecular volume. Hence, since 

 the critical temperature of marsh gas is less than —100°, and its crit- 

 ical pressure only 39 atmospheres, he believed he could approach the 

 absolute zero by its evaporation. In his paper he gives a valuable 

 table of the critical temperature, the critical pressure, and the ratio of 

 the two for 21 gases, in parallel columns. {Phil. Mag., September, 1884, 

 V, XVIII, 210.) 



Olszewski, using a hydrogen thermometer, has shown that under a 

 pressure of 1 atmosphere oxygen boils at —181°, and that when it evap- 

 orates under a pressure of 6""" of mercury the temperature is —198° 

 But even this latter temperature was not sufficiently low to liquefy hy- 

 drogen, being above its critical temperature. Recourse was therefore 

 had to nitrogen. This gas, under a pressure of 60 atmospheres, cooled 

 to —142° by ethylene boiling in vacuo, liquefied, but showed no meniscus. 

 On diminishing the pressure to 35 atmospheres, the nitrogen boiled so 

 rapidly that it appeared white and opaque in the upper part of the tube. 

 Keeping the pressure at this point, the boiling ceased, the liquid became 

 clear and showed a well-pronounced meniscus. About 3 or 4 c. c. of 

 liquid were obtained, which evaporated slowly, increasing the pressure 

 until it reached 30.2 atmospheres, the critical pressure, when the menis- 

 cus disax^peared. Exposed to the atmospheric pressure the liquid evap- 

 orated at first rapidly, then more slowly, the liquid remaining transpar- 

 ent, with no trace of the crystals observed by Wroblewski. Indeed, by 

 no sudden expansion could it be frozen. But when a sudden expansion 

 was produced with hydrogen in a tube placed within the liquid nitro- 

 gen, the pressure falling from 160 to 40 atmospheres, the hydrogen con- 

 densed as a colorless and transparent liquid, running down the walls of 

 the tube. An instant after, the outside of this tube became covered 

 with a white, opaque layer of solidified nitrogen, produced by the intense 

 cold given by the boiling hydrogen. Subsequently Olszewski employed 

 liquid ethylene under a pressure of 10'""' of mercury and obtained a tem- 

 perature of —150°. He studied the temperatures obtained at various 

 pressures, as follows: At 750™", —103°; at 546""', -105°; at 441""", 

 -108O; at316""", -llio; at 246'""', -115.50; at 146""", -122°; at 107""", 

 -126°; at 72'""', -129.7°; at 56™", -132°; at 31""', -139°; at 12"", 

 — 148° ; and at 9.8"", — 150.4°. At this latter temperature the liquefaction 

 of several cubic centimeters of nitrogen presents no difficulties, it being 



