RADIOACTIVITY 25 



the rate at which it disappears is proportional to the quantity 

 in existence. The life of the others is so short that their 

 growth cannot be observed in comparison with that of the 

 emanation and they seem to follow closely at its heels. In 

 fig. 8 is shown the form of the curve at different times after 

 the purification of the radium ; it will be seen how excellently 

 this surmise is fulfilled. The radium block is unaltered in form 

 as it moves slowly to the right : the others grow together 

 day by day and after a week or two the curve shows that the 

 normal condition of things has again been reached. 



lorvi^ariorv 



Fig. 8. 



The four curves in the figure going from left to right were obtained (i) directly after preparation of the 

 film, (2) after 64 hours, (3) after 140 hours, and (4) after 28 days respectively. See Phil. Mag., Sept. 

 1905, p. 323. 



All these experimental tests of Rutherford's theory, using 

 the newly discovered properties of the a rays, were made after 

 the main facts of the theory had been worked out by quite 

 different methods. It is therefore the very best testimony that 

 such a theory could have ; in every detail the effects observed 

 answered to such expectations as could be formed and where 

 they revealed fresh facts or laws they dovetailed in with the 

 old in a most satisfactory manner. Take for example the 



