﻿52 Mr. Norman Campbell on Delta Rays. 



electrodes are covered with soot cannot be greatly influenced 

 by reflexion ; its form must be determined on our present 

 ■view by a direct stopping of the rays by the opposing electric 

 field. But it has been noted that the current with soot- 

 covered electrodes is not saturated until a potential difference 

 of more than 40 volts is applied. The only possible inter- 

 pretation of this fact consistent with our present hypothesis 

 is that some of the rays emitted by soot have a speed corre- 

 sponding to a potential difference greater than 40 volts. 

 Now, w T hen A is covered with soot and B with aluminium, 

 we must suppose that rays with a speed corresponding to 

 40 volts are leaving A and striking B. Unless all these 

 rays are reflected from B (a supposition for which Baeyer's 

 work gives no warrant) the current obtained by raising A 

 to a positive potential cannot become saturated until that 

 potential is great enough to prevent the rays reaching B at 

 all, that is, until it is greater than 40 volts ; but experiment 

 show r s that the current is saturated when the potential of A 

 is 4- 10 volts. 



Or again, it will be seen that the positive branches of the 

 curves w T hen A is covered with soot are practically identical 

 whether B is covered with aluminium, copper, silver, or gold. 

 Even on the reflexion hypothesis this result implies that the 

 speed of the rays from those four metals is practically the 

 same, for the reflecting power undoubtedly depends upon 

 the speed of the incident rays. But, if the speed of: the 

 rays from these four substances is the same, it is utterly un- 

 justifiable to suppose that the speed of the rays from soot is 

 very different ; and, if the speed of the rays from aluminium 

 (say) is the same as the speed of the rays from soot, how 

 comes it that the current with aluminium electrodes is satu- 

 rated by 40 volts when the rays from soot undoubtedly 

 include some which have a speed greater than that corre- 

 sponding to 40 volts? I conclude that the hypothesis of 

 reflexion together with the simple theory of the last paper 

 is quite incapable of accounting for the results obtained 

 when one of the electrodes is covered with soot. 



6. It is easy to frame an alternative hypothesis for this 

 case. The main feature to be explained is that the branch of 

 the curve corresponding to positive potentials on B represents 

 the properties of A and not the properties of B. Hitherto 

 we have supposed that, in the absence of an electric field, 

 the delta rays leave readily the electrode at which they are 

 generated and that the action of the field is to prevent some 

 of the rays leaving it. If we make exactly the contrary 

 supposition, that in the absence of a field very few of the 



