300 Mr. Norman Campbell on Delta Rays. 



(3) The incidence radiation appears never to be less than 

 the emergence, though it is sometimes not greater. This 

 conclusion conflicts with that obtained by Kleeman (loc. cit.) 

 by an indirect method for gases. The difference is possibly 

 due to the fact that in a gas the positively charged atoms 

 from which the rays are emitted can move. 



(4) It is doubtful whether the speed of the delta rays 

 varies with the material from which they are emitted. Such 

 a variation seems to have been detected, but considerations 

 of the possible effect of the reflexion of the delta rays at the 

 electrodes renders the prima facie evidence unreliable. It 

 is worthy of note that the conclusion that the speed of the 

 delta rays varied neither with the speed of the alpha rays nor 

 with the material from which they are emitted, would be 

 intensely interesting. In that case there would be apparently 

 nothing left for them to vary with, and it would have to be 

 concluded that the speed of the rays was a universal constant. 

 Such a conclusion would not be inconsistent with the view 

 put forward by Sir J. J. Thomson, that the ionization of a 

 gas consists of the separation from the atoms of it of neutral 

 bodies, similar for all atoms, which subsequently break into 

 a positively and a negatively charged part. The view that 

 there is an intermediate step between the impact of an alpha 

 ray and the emission of an electron from the atom struck 

 has also been put forward by Prof. Bragg. 



(5) There is no evidence that the number of the delta rays 

 emitted depends on the material from which they are emitted. 

 The influence of adhering air-films makes small differences 

 difficult to detect. It is possible, indeed, that no experiments 

 have really been made on different materials, and that in all 

 cases only the air-film has been dealt with. But the clear 

 differences noted in § 13 between the observations with dif- 

 ferent metals seem to negative such a suggestion. 



(6) There appear to be true delta rays emitted from the 

 active substance emitting the alpha rays in excess of those 

 due to the impact of those alpha rays on the support. 



I should like to take this opportunity of expressing my 

 great gratitude to Prof. Bragg and the University of Leeds, 

 for their kindness in placing the resources of the physical 

 laboratory at my disposal. 



Note added July 7. 



Experiments performed in the interval between writing 

 the above paper and revising the proofs have altered materially 

 the aspect of the subject. It appears now that the theory of 



