420 Messrs. Stead and Gosslino- on Relative Ionization 



Experiments have been made with most of the six gases 

 over a considerable range o£ pressure, and it has been shown 

 that the critical point in the curve, for any given gas, occurs 

 at a voltage which is sensibly independent of the pressure. 

 The change of slope at the critical point is_, of course, dependent 

 on the gas pressure, and becomes less marked as the pressure 

 is diminished, but otherwise there is no alteration. This is 

 brought out clearlv in fig. 6. 



In all the experiments the valve used was highly exhausted 

 by means of a Graede rotary pump, followed by charcoal cooled 

 by liquid air. During the evacuation the valve was baked 

 at a temperature of about 400° 0. for at least an hour, and the 

 cold electrodes were subjected to sufficient electron bom- 

 bardment to raise them to a red heat, in order to remove 

 occluded gas. The gas to be experimented upon was then 

 introduced in as pure a state as possible. The argon used 

 was prepared from liquid air residues by sparking with 

 oxygen over caustic potash, the excess of oxygen being 

 finally removed by means of phosphorus. The argon when 

 examined spectroscopically appeared to be free from nitrogen. 

 Hydrogen was prepared by the electrolysis of a solution of 

 barium hydroxide in water. Nitrogen from air was tried, 

 but for most of the experiments it was prepared by mixing 

 strong solutions of ammonium chloride and sodium nitrite 

 and warming the mixture. Carbon monoxide was obtained 

 by the action of strong sulphuric acid on formic acid, and 

 also by heating nickel carbonyl. The helium used was 

 obtained from Thomas Tyrer & Co., Ltd., and was purified 

 by being left in contact with charcoal cooled by liquid air 

 for a considerable time. In all cases mercury vapour was 

 excluded by placing between the valve and the pump a 

 U-tube immersed in liquid air. When mercury vapour 

 itself was being studied this liquid air was removed after the 

 valve had been thoroughly evacuated. 



It may be remarked that, from the point of view of expe- 

 riments on valves, and other apparatus in which a hot cathode 

 is used, a knowledge of ionization potentials is often of great 

 serviee, as the presence of a given gas in a sealed valve may 

 frequently be detected by measuring the ionization potential 

 in this way. Moreover, the test is quite sensitive. Thus the 

 presence of mercury vapour is readily shown, even when it 

 cannot be detected spectroscopically. Of course the con- 

 ditions in a valve are not good for spectroscopic observation, 

 as the discharge obtained through the bulb by means of an 

 induction-coil is not very bright, but, nevertheless, this gives 

 some idea of the sensitiveness of the ionization test. Again, 



