282 Mr Campbell, 



The Radio-activity of Metals and their Salts. By Norman R. 

 Campbell, M.A., Trinity College. 



[Bead 12 March 1906.] 



1. Considerable evidence has accumulated in favour of the 

 view that all elements are truly radio-active. It has been shown 

 that ordinary metals emit a radiation analogous to that emitted 

 by radium, and that the properties of this radiation are character- 

 istic of the metal from which it proceeds. But there is a 

 remarkable property of radio-activity which suggests a question 

 that may be asked and can be answered before the source of the 

 activity of ordinary materials can be regarded as established 

 beyond doubt. 



It is known that the activity of radium and uranium is an 

 atomic property : the activity of any substance containing these 

 elements is independent of its chemical and molecular condition, 

 and depends only on the quantity of these elements which it 

 contains. Is this law true for ordinary materials ? Could we 

 predict (say) the activity of lead oxide from the known activities 

 of lead and oxygen ? The experiments described below are an 

 attempt to answer this question. 



We need only take into account the a radiation, for the analogy 

 of radium leads us to suppose that the energy contained in any /3 

 or 7 rays that may be emitted will be infinitesimal compared to 

 that carried by the a rays. 



2. It has been shown* that the ionisation (p) caused by the 

 a radiation from an infinitely thick slab containing a radio-active 

 substance is given by 



p = CkaA, 



where C is a constant, k the mass of the radiating material con- 

 tained in unit mass of the slab, a Bragg's constant for the rays 

 emitted by that material and A a quantity measuring its activity 

 in terms of the energy given off from it. The activity (p uvw ...) of 

 a slab composed entirely of several radiating elements u, v, w, etc. 

 will be given by 



'Puvw • • • == ^ \"'uQ"u-**-u t" n v a v Jx v -j- K w a w Ji w -+■ ... ), 

 where the suffixes denote the element to which each quantity 

 refers. Now the ionisation caused by a slab composed entirely of 

 one of the elements (u) will be given by the relation 



p u = L>a u A. u , 

 * Phil. Mag. Feb. 1906, p. 210. 



