RADIOACTIVITY 21 



fortunately the simple supposition may be made that they all 

 have the same initial speed. Or again it may be supposed, as I 

 originally did, that the radium layer has an appreciable thickness 

 and that the particles from the bottom of it lose some of their 

 speed on the way out. This turns out to be so far right that if 

 the thickness of the layer be increased the slope from p to q 

 becomes longer ; that is to say, there are particles which have 

 come from so far down in the radium that they have not velocity 

 enough to get so far as 65 cm. ; indeed, if the layer be thick 

 enough there are some that can only just emerge. But even 

 when the layer is reduced to the extremest thinness there is still 

 a slope at p q ; we can never get the curve to be quite as sharp 

 as in fig. 4. Probably the explanation follows from results 

 obtained by H. Geiger, who shows that when the particles are 

 near the end of their course they are somewhat knocked about 

 and thrown into paths which end at varying distances from the 

 source. We will consider Geiger's work later. 



Let us go on with the experiment. Suppose the radium is 

 pushed somewhat closer, up to a distance of 4/8 cm., the effect 

 of the rays in the chamber goes on steadily diminishing and 

 then suddenly it increases again. The chamber has evidently 

 come within the range of another set of projectiles and we find 

 the same phenomena repeated. Again there is a rapid increase 

 for an approach of about half a centimetre shown by the portion 

 r s ; there is a pause : and then (see fig. 5), just as we are 

 apparently beginning to find the decrease corresponding to q r, 

 a third stream is struck and the phenomena are repeated a 

 third time. This time the curve does get a little way round the 

 bend, as at t u ; then almost immediately a fourth set of particles 

 is struck and for the last time we have a slope u v corre- 

 sponding to the first slope p q and a final return of the curve 

 vw. Thus there appear to be four sets of a particles. 



When this quadruple effect was first observed, its complete 

 correlation with Rutherford's theory was obvious. Rutherford 

 had shown, as already stated, that from radium proceed other 

 substances in successive derivation, the which substances are also 

 radioactive. The exact genealogy is as follows : Radium itself 

 has a low mortality or, as we may say, a long average life ; in 

 fact, the average life is now supposed to be about twenty 

 centuries. Its immediate descendant is the emanation which 

 has a far shorter life, for its atoms only live some three and 



