Liquefaction of Gases. 203 



pressure, viz., approximately at 18, 16, and 14 atm. I 

 repeated the same experiment a good many times, and always 

 obtained the same results. These experiments bring me to 

 the conclusion, that the 20 atm. at which the ebullition of 

 hydrogen always appears represents its critical pressure. If 

 hydrogen, cooled by means of liquid oxygen, boiling in vacuo, 

 to the temperature — 211° C, which, we may suppose, is 

 several degrees above the critical temperature of hydrogen, is 

 submitted to a slow expansion from a high pressure, its 

 temperature is lowered to the critical temperature, hitherto 

 unknown. If the initial pressure is high enough — in my 

 experiments it was above 80 atm. — then, by means of a slow 

 expansion, the temperature of hydrogen sinks to its critical 

 value, before its critical pressure is reached, and then liquid 

 hydrogen will appear the moment we lower the pressure to its 

 critical value. But if the initial pressure is too low, a slow 

 expansion cools the hydrogen to the critical temperature only 

 after the critical pressure has been passed : the lower the 

 initial pressure is the greater is the expansion needed to cool the 

 hydrogen below its condensing temperature. We may thus 

 explain the changing pressures, corresponding to the phe- 

 nomenon of ebullition or instantaneous liquefaction in the case 

 of expansion from an insufficient initial temperature. And if 

 the initial pressure is still lower, the instantaneous liquefaction 

 will not appear at all. 



To ascertain the truth of this statement I performed two 

 series of analogous experiments with gases, the critical 

 pressures and temperatures of which are accurately knowix, 

 viz., with oxygen and ethylene. The critical temperature of 

 oxygen is, according to my former researches, — 118°'8C, 

 its critical pressure is 50'8 atm. In the same apparatus 

 which I used for the experiments with hydrogen I cooled 

 oxygen by means of ethylene boiling under atmospheric 

 pressure ( — 102°*5), then to a temperature 16*3 degrees below 

 the critical temperature of oxygen, and subjected it to a 

 slow expansion, beginning with different initial pressures, 

 from 40 atm. up to 100 atm. The ebullition of oxygen 

 always appeared at a pressure of about 51 atm., provided the 

 initial pressure was not lower than 80 atm. : at the same 

 time there also appeared a meniscus of liquid oxygen. As 

 the initial pressure became lower and lower, so did the ebulli- 

 tion pressure too. 



The critical temperature of ethylene according to Prof. 

 Dewar is 10 o, l, the critical pressure 51 atm. ; my own deter- 

 minations of the same quantities yielded results agreeing well 

 with the above-cited, viz., 10° C. and 51*7 atm. 



