204 Prof. C. Olszewski on the 



I made similar experiments with ethylene, using the appa- 

 ratus of Cailletet ; one series at a temperature of 17° C, 

 another at 27°; then at temperatures, which were first 7°, then 

 17° higher than the critical temperature of ethylene. During 

 the first series of experiments, the ebullition of ethylene, and 

 at the same time the meniscus, appeared constantly in conse- 

 quence of a slow expansion at a pressure of about 51 atm., if 

 the initial pressure was 70, 80, 90, 100, and 110 atm. During 

 the second series of experiments the ebullition appeared at the 

 same pressure, if the initial pressure was 100, 110, 120, and 

 130 atm. In proportion as the initial pressure was lowered — 

 in the first series below 70 atm., in the second below 100 

 atm. — the ebullition pressure was lowered too. I must, 

 however, mention that in the apparatus of Cailletet, in which 

 I made the experiments with ethylene, the conditions of ebulli- 

 tion of any gas by expansion are much less advantageous than 

 in the apparatus described above. 



Hence it follows that the determination of critical pressures 

 by means of expansion is possible, even if the gases have a 

 temperature which is several or many degrees higher than 

 their critical temperature. This dynamical method of deter- 

 mination of critical pressure is really of no advantage if applied 

 to the other gases, for these pressures may be more easily and 

 precisely determined by the vanishing of the meniscus ; but 

 with hydrogen it is the sole possible way to determine not 

 only its critical pressure, but also its critical temperature. 



On repeating these experiments in November 1891 I used 

 liquid air, boiling under a pressure of 4-10 millim., as a cooling 

 agent, and obtained the same results, with the only difference 

 that the ebullition of hydrogen on expansion appeared still 

 more distinctly and persisted somewhat longer. 



The reason for which it has not been hitherto possible to 

 liquefy hydrogen in a static state, is that there exists no gas 

 having a density between those of hydrogen and of nitrogen, 

 and which might be for instance 7-10 (H = l). Such a gas 

 could be liquefied by means of liquid oxygen or air as cooling 

 agent, and be afterwards used as a frigorific menstruum in the 

 liquefaction of hydrogen. 



The subjoined figure 3, taken from the original, repre- 

 sents my apparatus for liquefying large quantities of oxygen 

 and air, connected with the apparatus serving to deter- 

 mine the critical pressure of hydrogen. The following brief 

 description will help to understand the figure : — 



(a) The steel cylinder, 200 cub. centim. in capacity, for the 

 liquefaction of oxygen or air. 



(b) The glass vessel with triple walls, serving to receive 

 liquid ethylene under diminished pressure. 



