3 o POPULAR SCIENCE MONTHLY. 



tion. In order to account for the exjDulsion of an a and a /? particle 

 with the observed velocities, it is necessary to suppose that the particles 

 are in a state of rapid motion in the system from which they escape. 

 Variation of temperature;, in most cases, does not seem to affect the 

 stability of the system. 



It is well established that the property of radioactivity is inherent 

 in the radio-atoms, since the activity of any radioactive compound de- 

 pends only on the amount of the element present and is not affected 

 by chemical treatment. As far as observation has gone, both uranium 

 and radium behave as elements in the usually accepted chemical sense. 

 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 com- 

 pounds. 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 evidently suggest that the heavier inactive ele- 

 ments may also prove to be composite. 



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 1,000 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 in uranium and thorium, it is to be 

 expected that a measurable proportion of the radium will be trans- 

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

 gradually disappear as such in consequence of its gradual transforma- 

 tion into other substances. This conclusion necessarily follows from 

 the known experimental facts. The radium is being transformed 

 continuously into the emanation which in turn is changed into other 

 types of matter. Since there is no evidence that the process is re- 

 versible, all the raduim present must, in the course of time, be trans- 

 formed into emanation. The rate at which radium is being trans- 

 formed 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 showing that in a gram of radium about half a milligram is trans- 

 formed per year. From analogy with other radioactive changes, it is 



