Phenomena in X-Pay Bulbs. 203 



would be re-established, and this is in good agreement with 

 the results. The fact that a bulb is polarized in only one 

 direction may be explained by supposing that the action 

 of electronic bombardment on the anode is not sufficient 

 to destroy the gaseous layer on its surface. The effect of 

 water-vapour mentioned in § 4 shows that this gas is 

 capable of forming a layer on metal surfaces with much 

 greater ease than other gases. 



It is possible that the presence of gaseous layers on the 

 electrodes also plays an important part in their absorption of 

 the electric charges which are carried by the ions. The 

 well-known phenomenon of double electric layers on metal 

 surfaces shows that an encounter between an ion and a metal 

 surface is not necessarily followedrby the transference of the 

 electric charge from the former to the latter. It is possible 

 that when the gaseous layer on the cathode is disintegrated,. 

 the positive ions striking against it are unable to impart their 

 charges to it, but rather form a positive electric layer very 

 close to its surface, and that this layer causes the polarization 

 of the bulb. This assumption could be tested experimentally ; 

 for, firstly, the cathode fall of potential would be expected 

 from this point of view to remain constant, or even to 

 increase when the state of polarization sets in and a high 

 p.d. put across the bulb fails to produce a discharge through 

 it. The theory may also be checked by introducing into the 

 bulb a heated platinum strip coated with a salt emitting 

 positive ions; for, if then the bulb be polarized and a p.d. 

 established between the cathode and the hot platinum strip 

 as an anode, the positive emission from the strip should not 

 be expected to produce a current through the bulb. Experi- 

 ments in this direction are now in progress. 



Hardness of a Polarized Bulb. 



8. As mentioned above, after a sufficiently long run the 

 beam of X-ray emitted by the bulb exhibits a well-marked 

 tendency to increase in penetrating power in spite of. the 

 maintenance of comparatively high pressures, and when the 

 bulb is polarized, very penetrating rays may be produced at 

 pressures as high as -j 1 - mm. The experiments show, in 

 general, that the hardness of the rays is determined entirely 

 by the potential difference and is independent of the pressure, 

 which is not a little surprising if we bear in mind that under 

 the conditions of the experiments the electrons had to 

 undergo a considerable number of collisions with the gaseous 

 molecules before reaching the anode. It must be assumed 



