RADIOACTIVITY 23 



into existence in any second is equal of course to the number 

 disintegrating in the same time and producing the next in order. 

 Consequently there must be equal numbers in each of the four 

 a particle streams. Now, if we examine by scale the curve in 

 fig. 5 we find that it is compounded of four curves exactly the 

 same in every respect but that of vertical height. This is just 

 as it should be, if we make the further supposition that the a 

 particles are all alike though they are the product of different 

 radioactive substances. 



If we could isolate for a sufficient time each one of the radio- 

 active derivatives of radium and make a thin layer of it upon a 

 plate to be then placed in our apparatus, we should no doubt be 

 able to see its simple curve. We should learn which of the 

 radioactive substances gives rise to each of the different sets of 

 rays : we should know the " range " of the a ray from each 

 substance and be able to use it as a distinguishing characteristic. 

 It is just possible to do this but with difficulty; and we may con- 

 sider other experiments which, though a little less easy of 

 interpretation, are much easier to perform. Let us remind 

 ourselves that the emanation, radium A, B and C are all different 

 substances and can be separated by physical or chemical 

 treatment. 



For example, the emanation is easily driven off from the 

 radium by the application of heat. Let us heat to redness the 

 plate on which the radium rests, replace it in the range-finding 

 apparatus and remeasure the ionisation effects. We now 

 obtain the curve as represented in fig. 6. By comparison with 

 fig. 5 we see that the two centre blocks have disappeared. But 

 why two ? We should have expected the effects of the 

 emanation to have been removed but we see that a second 

 has gone at the same time. There is of course a ready answer 

 which does not require us to suppose R A or R C to be readily 

 volatilised. It is that Ra A has a life of only three or four 

 minutes. When therefore we drive off the emanation by heat 

 and the origin of RaA is removed, the store of RaA is soon 

 used up and we have not been quick enough in our experimental 

 arrangements to catch the effects of the disappearing product. 

 Indeed, unless we are fairly quick we shall not see much of the 

 effect of the Ra C either, for its average life is under half an 

 hour; and in fact the form of the curve alters as we measure 

 it. In a very few hours it takes the form of fig. 7 : the top block 



