536 Dr. Norman Campbell : Further 



should appear always of the same quality. It seems to me 

 sufficient evidence against this view that different materials 

 obviously reflect the rays differently: if the rays can penetrate 

 the air film (from which doubtless some rays come) when 

 they are incident on the material, surely some of the rays 

 excited in the material must be able to emerge. It is just 

 possible, however, that the differences which have been 

 observed with different materials are caused only by a 

 difference in the state of the air film on their surfaces. But 

 I have never found that the most drastic treatment for 

 removing air films has produced any considerable change in 

 the form of the curve. 



Secondly, we may ask, What is the velocity of the delta 

 rays? I think it is clear from these experiments that there 

 is no definite velocity and that rays are emitted with widely 

 different speeds ; some, probably about half of them, seem to 

 have a velocity of less than 1 volt, while others seem to have 

 speeds as great as 10 or 20 volts. It is to be remembered 

 that the rays are emitted at all angles with the surfaces of 

 the electrode, whereas it is only the component of their 

 velocity perpendicular to the electrodes which is effective in 

 carrying them across. But this fact is not sufficient to 

 account for the apparent heterogeneity of the rays. It is 

 easy to show that if the rays were emitted with the same 

 speed of V volts, but in all directions, we should have (when 

 V lies between and V ) 



ip-iv _ / vy 



to -\Vo/' 



The curve should touch the axis of potential at V = 0, and 

 should have a marked " knee " at the point where saturation 

 is attained ; the actual curves exhibit neither of these 

 features. 



On the other hand, heterogeneity of the rays may possibly 

 be due to a loss of velocity by those rays which are not 

 produced actually in the surface in passing through the inter- 

 vening layer. Since the rays are completely stopped by the 

 thinnest material layers obtainable, it seems impossible to 

 test this hypothesis. 



12. Some further experiments have been made with the 

 apparatus described in the paper on " Ionization by Alpha 

 Rays " (loc. cit.) on the current through a. layer of gas at 

 low pressure contained between parallel electrodes and 

 ionized by alpha rays. The electrodes were covered with 

 aluminium ; in one series they were about i mm. apart, in 



