1 8 POPULAR SCIENCE MONTHLY. 



activity lasts, should be of the same order of magnitude, but in the 

 case of uranium and thorium the present rate of heat emission will 

 probably continue, on an average, for a period of about 1000 million 

 years. 



Source of the Energy emitted by the Radioactive Bodies. 



There has been considerable difference of opinion in regard to the 

 fundamental question of the origin of the energy spontaneously 

 emitted from the radioactive bodies. Some have considered that 

 the atoms of the radio-elements act as transformers of borrowed en- 

 ergy. The atoms are supposed, in some way, to abstract energy from 

 the surrounding medium and to emit it again in the form of the 

 characteristic radiations. Another theory which has found favor with 

 a number of physicists supposes that the energy is derived from the 

 radio-atoms themselves and is released in consequence of their dis- 

 integration. The latter theory involves the conception that the atoms 

 of the radio-elements contain a great store of latent energy, which only 

 manifests itself when the atom breaks up. There is no direct evidence 

 in support of the view that the energy of the radio-elements is de- 

 rived from external sources, while there is much indirect evidence 

 against it. Some of this evidence will now be considered. There is 

 now no doubt that the a and /? rays consist of particles projected with 

 great speed. In order that the a particle may acquire the velocity with 

 which it is expelled, it can be calculated that it would be necessary for 

 it to move freely between two points differing in potential by about 

 five million volts. It is very difficult to imagine any mechanism, 

 which could suddenly impress such an enormous velocity on one of the 

 parts of an atom. It seems much more reasonable to suppose that the 

 a and /? particles were originally in rapid motion in the atom and, 

 for some reason, escaped from the atomic system with the velocity they 

 possessed at the instant of their release. There is now undeniable 

 evidence that radioactivity is always accompanied by the production of 

 new kinds of active matter. Some sort of chemical theory is thus 

 required to explain the facts whether the view is taken that the energy 

 is derived from the atom itself or from external sources. The ' ex- 

 ternal ' theory of the origin of the energy was initially advanced to 

 explain only the heat emission of radium. We have seen that this is 

 undoubtedly connected with the expulsion of a particles from the 

 different disintegration products of radium, and that the radium itself 

 only supplies one quarter of the total heat emission, the rest being 

 derived from the emanation and its further products. On such a 

 theory it is necessary to suppose that in radium there are a number 

 of different active substances, whose power of absorbing external energy 

 dies away with the time, at different but definite rates. This still 

 leaves the fundamental difficulty of the origin of these radioactive 



