212 Prof. J. Dewar on the Liquefaction of Oxygen 



respective tubes. When the pump has reduced the pressure 

 to 25 mm., the ethylene has a temperature of about 

 — 140° C; a pressure of between 20 and 30 atmospheres 

 is then sufficient to produce liquid oxygen in the tube F. 

 The tube F is 5 mm. in diameter and about 3 mm. thick in 

 the walls, and when filled with fluid oxygen (for projection) 

 holds at least 1*5 cubic centim. With such a quantity of 

 fluid oxygen it is easy to show its ebullition at ordinary pres- 

 sures, and by means of a thermo-junction to demonstrate the 

 great reduction of temperature which is attendant on its 

 change of state at atmospheric pressure. 



Provided a supply of liquid ethylene can be had, there is no 

 difficulty in repeating all the experiments of the Russian obser- 

 vers ; but as this gas is troublesome to make in quantity, and 

 cannot be bought like carbonic acid or nitrous oxide, such expe- 

 riments necessitate a considerable sacrifice of time. It was 

 therefore with considerable satisfaction that I observed the pro- 

 duction of liquid oxygen by the use of solid carbonic acid, or 

 preferably liquid nitrous oxide. When these substances are 

 employed and the pressure is reduced to about 25 mm., the tem- 

 perature of — 115° C. may be taken as that of the carbonic acid, 

 and —125° C. as that of the nitrous oxide. As the critical 

 point of oxygen, according to the Russian observers, is 

 about —113° C, both these cooling agents may be said to 

 lower the temperature sufficiently to produce liquid oxygen, 

 provided a pressure of the gas above the critical pressure, 

 which is 50 atmospheres, is at command. In any case, how- 

 ever, the temperature is near that of the critical point ; and 

 as it is difficult to maintain the pressure below about an inch 

 of mercury, the temperature is apt to be rather above the 

 respective temperatures of —115° C. and —125° C. In order 

 to get liquefaction conveniently with either of these agents, it 

 is necessary to work at a pressure of oxygen gas from 80 to 

 100 atmospheres, and to have the means of producing a 

 sudden expansion when the compressed gas is cooled to the 

 above-mentioned temperatures. This is brought about by the 

 use of an additional stopcock, represented in the figure at B. 

 During the expansion the stopcock at A is closed and the pres- 

 sure-manometer carefully observed. No doubt liquid nitrous 

 oxide is the most convenient substance to use as a cooling agent; 

 but as it is apt to get superheated during the reduction of 

 pressure and boil over with explosive bursts of vapour, it is well 

 to collect the fluid in a small flask of about 250 cub. centim. 

 capacity, and to change it into the solid state by connecting 

 the flask with the air-pump, and then to use the substance in 

 this form. The addition of alcohol or ether to the solid nitrous 



