August 27, 1909] 



SCIENCE 



277 



different atoms of a radioactive substance 

 are not, in all respects, identical. 



The energy developed by radioactive sub- 

 stances is exceedingly large, one gram of 

 radium developing neai-ly as much energy 

 as would be produced by burning a ton of 

 coal. This energv' is mainlj^ in the a par- 

 ticles, the positively charged helium atoms 

 which are emitted when the change in the 

 atom takes place; if this energy were pro- 

 duced by electrical forces it would indicate 

 that the helium atom had moved through 

 a potential difference of about two million 

 volts on its way out of the atom of radium. 

 The source of this energy is a problem of 

 the deepest interest; if it arises from the 

 repulsion of similarlj' electrified systems 

 exerting forces varying invei-sely as the 

 square of the distance, then to get the requi- 

 site amount of energy the systems, if their 

 charges were comparable with the charge 

 on the a particle, could not when they start 

 be further apart than the radius of a cor- 

 puscle, 10"" cm. If we suppose that the 

 particles do not acquire this energy at the 

 explosion, but that before they are shot out 

 of the radium atom they move in circles in- 

 side this atom with the speed with which 

 they emerge, the forces required to prevent 

 particles moving with this velocity from 

 flying off at a tangent are so great that 

 finite charges of electricity col^ld only pro- 

 duce them at distances comparable with 

 the radius of a corpuscle. 



One method by which the requisite 

 amount of energy could be obtained is sug- 

 gested by the view to which I have already 

 alluded— that in the atom we have electri- 

 fied systems of very different types, one 

 small, the other large; the radius of one 

 type is comparable with 10"" cm., that of 

 the other is about 100,000 times greater. 

 The electrostatic potential energy in the 

 smaller bodies is enormously greater than 

 that in the larger ones; if one of these 



small bodies were to explode and expand to 

 the size of the larger ones, we should have 

 a liberation of energy large enough to en- 

 dow an a particle with the energy it pos- 

 sesses. Is it possible that the positive units 

 of electricity were, to begin with, quite as 

 small as the negative, but while in the 

 course of ages most of these have passed 

 from the smaller stage to the larger, there 

 are some small ones still lingering in radio- 

 active substances, and it is the explosion of 

 these which liberates the energy set free 

 during radioactive transformation ? 



The properties of radiiim have conse- 

 quences of enormous importance to the 

 geologist as well as to the physicist or chem- 

 ist. In fact, the discovery of these proper- 

 ties has entirely altered the aspect of one 

 of the most interesting geological problems, 

 that of the age of the earth. Before the 

 discovery of radium it was supposed that 

 the supplies of heat furnished by chemical 

 changes going on in the earth were quite 

 insignificant, and that there was nothing to 

 replace the heat which flows from the hot 

 interior of the earth to the colder crust. 

 Now when the earth first solidified it only 

 possessed a certain amount of capital in the 

 form of heat, and if it is continually spend- 

 ing this capital and not gaining any fresh 

 heat it is evident that the process can not 

 have been going on for more than a certain 

 number of years, otherwise the earth would 

 be colder than it is. Lord Kelvin in this 

 way estimated the age of the earth to be 

 less than 100 million yeai-s. Though the 

 quantity of radium in the earth is an ex- 

 ceedingly small fraction of the mass of the 

 earth, only amounting, according to the de- 

 terminations of Professors Strutt and Joly, 

 to about five grams in a cube whose side is 

 100 miles, yet the amount of heat given out 

 by this small quantity of radium is so great 

 that it is more than enough to replace the 

 heat which flows from the inside to the 



