Ionization of Platinum by Cathode Rays. 299 



the views which are prevalent as to the contamination 

 of metal surfaces with gas unless they are specially purified. 

 The contamination (which we shall suppose to consist of 

 hydrogen, partly on the ground of the value of W, partly 

 on the ground of other work) probably is present in two forms. 

 First there is gas condensed on the surface of the metal, 

 covering it completely; second, there is gas dissolved or 

 entangled in someway in the body of the metal. The change 

 from state A to state B may be supposed to represent the 

 abolition of the surface layer, the " dissolved gas " being 

 practically untouched ; while the gradual decrease in Z after 

 state B is attained represents a diminution of the u dissolved 

 gas" and a diminution of the surface of it exposed to the 

 rays. If it be supposed, further, that 11 volt rays ionize only 

 the gas and not the metal, the main features of the observations 

 will be explained in a very simple and obvious way. In order 

 to explain the difficulty of restoring state A permanently and 

 the ease with which state B, once attained, may be regained, 

 we may imagine that in order to liberate the surface gas 

 some alteration in the mechanical structure of the surface 

 must be made; and that after it is made it is not easily 

 reversed. There is some evidence for such an idea in the 

 change of R w; '. When state A was completely restored by 

 the method of § 10, the value of B.J (in the state B) was not 

 restored. When state B was first attained R OT ' was 0'44, 

 the original value in state A being 0*50. Before state A 

 was restored, R m ' had sunk to 0*30; when state A was 

 restored, it rose to 0*52, its original value ; but when state B 

 was established once more, it returned to 0*33 and not to the 

 value it had when state B was first attained. 



17. The temporary changes produced by the electric dis- 

 charge indicate/ probably the effect of the surface "double 

 layers," studiea recently by Seeliger*. According to his 

 results, P, except just after it has been made the cathode of a 

 discharge, will he covered with a double layer, the potential 

 in which is 3 volts with the positive side outwards. 



If such slow rays as are used in these experiments are capable 

 of penetrating the double layer at all, its presence will, of 

 course, have no effect on the speed with which they reach the 

 metal. But the double layer will help electrons liberated at 

 its inner surface to escape. Researches on delta rays have 

 shown that these slow electrons are undoubtedly aided in 

 their escape from a solid surface by a favourable field. The 

 abolition of the layer by the discharge would remove the field 

 temporarily, fewer electrons would escape and Z would 

 * Seeliger, Phys. Zeit. xiv. p. 1273 (1913). 



