1905.] on New Low Temperature Phenomena. 185 



The gas, which was rapidly expelled, measured 5 • 7 litres, and 

 contained 56 per cent, of oxygen. If the saturated charcoal before 

 heating was subjected for an hour to the action of an air-pump, 

 capable of giving a steady exhaustion of 5 mm., no difference was 

 effected in the oxygen percentage of the evolved gas. The same 

 experiment was repeated with this variation, that, instead of the air 

 current having the pressure of the atmosphere, it was kept below 

 one-tenth of an atmosphere. In this experiment 4*8 litres were 

 expelled on heating up, and the percentage of oxygen was 58. 

 Then, a further repetition was made with an air current supplied at a 

 pressure not exceeding 5 mm. of mercury. After 3 hours' treatment, 

 the charcoal, on heating to 15° C, gave 4^ litres of 57 per cent, 

 oxygen. From these experiments it follows that the tension of the 

 occluded gases, at the temperature of liquid air, must be very small, 

 and thus the use of low temperatures, combined with charcoal, intro- 

 duces a new and greatly improved means of getting high vacua, which 

 in the future may be found susceptible of important practical applica- 

 tions. These experiments are quite conclusive as to the practical con- 

 stancy of the mean composition of the air gases occluded in the 

 charcoal (subject to the conditions aforesaid), and they further show 

 that wide changes in the pressure of the air current has little or no 

 effect in altering the proportions. In another experiment, the vessel 

 containing the saturated charcoal, instead of being allowed to rise 

 rapidly in temperature, was transferred to a vacuum vessel, in which 

 a little liquid air was placed, in order that the temperature might rise 

 slowly, and thereby enable the successive litres of gas given off to be 

 collected separately and analysed. 



This experiment gave the following results : — 



Oxygen per cent. 



First litre 18-5 



Second ,, 20*6 



Third 



Fourth 



Fifth 



Sixth 



53-0 

 720 

 79-0 

 84-0 



The mean composition of the 6 litres is again 56 per cent, oxygen. 

 From the above experiments it follows that one of the most rapid 

 means of extracting a high percentage of oxygen from atmospheric air 

 is to absorb it in charcoal at low temperatures, and thus to expel it 

 either rapidly or slowly by heating the mass of charcoal to the ordinary 

 temperature. 



A few experiments have been made using, instead of air, special 

 mixtures of oxygen and nitrogen. Thus it was found that a gas con- 

 taining 6 • 5 per cent, of oxygen, used in the same manner as in the air 

 occlusion experiments, gave, on heating up the charcoal rapidly to 

 15° C, 5 litres of gas having the composition of 23 per cent, of oxygen. 

 A repetition of the same process with the 23 per cent, of oxygen 

 would have raised the percentage about 60 per cent., or a stronger 



