182 PHYSICS OF THE ELECTRON 



They spontaneously break up, but the rate of their disintegration 

 seems to be, in most cases, quite independent of chemical control. 

 In this respect, the radioactive bodies occupy a unique position. It 

 seems reasonable to suppose that while the radioactive substances 

 behave chemically as elements, they are, in reality, compounds of 

 simpler kinds of matter, held together by much stronger forces than 

 those which exist between the components of ordinary molecular 

 compounds. Apart from the property of radioactivity, the radio- 

 elements do not show any chemical properties to distinguish them 

 from the non-radioactive elements, except their very high atomic 

 weight. The above considerations thus evidently suggest that the 

 heavier inactive elements may also prove to be composite. 



IX. Origin of the Radio-Elements 



We have seen that the radio-elements are continuously breaking 

 up and giving rise to a succession of new substances. In the case of 

 uranium and thorium, the disintegration proceeds at such a slow rate 

 that in all probability a period of about 1000 million years would be 

 required before half the matter present is transformed. In the case of 

 radium, however, where the process of disintegration proceeds at over 

 one million times the rate of uranium and thorium, it is to be expected 

 that a measurable proportion of the radium should be transformed 

 in a single year. A quantity of radium left to itself must gradually 

 disappear as such in consequence of its gradual transformation into 

 other substances. This conclusion necessarily follows from the known 

 experimental facts. The radium is continuously being transformed 

 into the emanation which in turn is changed into other types of 

 matter. Since there is no evidence that the process is reversible, 

 all the radium present must, in the course of time, be transformed 

 into emanation. The rate at which radium is being transformed can 

 be approximately calculated either from the number of a particles 

 expelled per second or from the observed volume of the emanation 

 produced per second. Both methods of calculation agree in fixing 

 that in a gram of radium about one milligram is transformed per 

 year. From analogy with other radioactive changes, it is to be 

 expected that the rate of change of radium would be always propor- 

 tional to the amount present. The amount of radium would thus 

 decrease exponentially with the time, falling to half value in about 

 1000 years. On this point of view, radium behaves in a similar way 

 to the other known products, the only difference being that its rate 

 of change is slower. We have already seen that, in all probability, 

 the product radium D is half transformed in about 40 years and ra- 

 dium E in about one year. In regard to their rate of change, the two 

 substances radium D and E, which are half transformed in about 40 



