930 Research Staff of the G. E. C, London, on the 



rather than by the temperature of the cathode ; the process 

 differed from that associated with the glow only in being 

 much slower. 



The question was therefore raised at what pressure the 

 absorption ceases. If absorption is due to ionization, it 

 should continue indefinitely, even at the very lowest pressures, 

 so long as the applied potential is greater than that required 

 for ionization. Experiments, which need not be described 

 in detail, confirmed this view. The absorption of gas in the 

 high-voltage lamps which were used in the main experiments 

 does not cease when the glow disappears and when the 

 bridge of Par. -4 shows a complete " clean-up/' If an 

 ionization gauge, adapted for very low pressure, is used, the 

 pressure will be found to decrease continually, but with 

 continually decreasing speed, until the limit of the 

 gauge is reached. If the usual precautions against the 

 evolution of gas from the metal parts are adopted, a vessel 

 can be evacuated until it shows the characteristics of 

 Langmuir's " high vacuum " by simply connecting it to an 

 ordinary incandescent lamp burnt at its normal voltage ; but 

 the exhaustion, even if the volume of the vessel is small 

 compared with that of the lamp, may take several days. 



On the other hand, no indication has ever been found of 

 absorption at potentials too small to cause ionization. 

 Experiments on this matter must be made on nitrogen, 

 on account of the peculiar property of hydrogen in dis- 

 appearing without ionization (cf. II., p. 686). It appears 

 that ionization of the gas, and possibly of the phosphorus, 

 is the one condition necessary for the absorption to occur. 

 Some abortive attempts have been made to establish this 

 proposition more certainly by ionizing a gas at atmospheric 

 pressure by ionizing rays ; but the proportion of the 

 molecules that can be ionized in any finite time by such a 

 process is so small that success was hardly to be expected. 



Destination of the Gas. 



15. Though all the evidence goes to prove that chemical 

 action in the ordinary sense plays at most a secondary part 

 in the absorption, it was thought desirable to trace as far as 

 possible any chemical changes that take place. The gases 

 in the lamps used in the absorption experiments were analysed 

 by the method recently described by us *. The only gases 

 which have ever been identified in the lamps, besides 



* Research Staff of G. E. C, London. Phys. Soc. Proc. xxxiii. p. 287 

 (1921). 



