590 On Radioactive Change. 



is therefore about '015 gram-calorie per year, Dividing this 

 quantity by the total energy of radiation, 2'4 10 7 gram-calories, 

 we obtain the number 6 10 -10 as a maximum estimate for 

 the proportionate amount of uranium or thorium undergoing 

 change per year. Hence in one gram of these elements, less 

 than a milligram would change in a million years. In the 

 case of radium, however, the same amount must be changing 

 per gram per year. The " life " of the radium cannot be 

 in consequence more than a few thousand years on this 

 minimum estimate, based on the assumption that each particle 

 produces one ray at each change. If more are produced the 

 life becomes correspondingly longer, but as a maximum the 

 estimate can hardly be increased more than 50 times. So 

 that it appears certain that the radium present in a mineral 

 has not been in existence as long as the mineral itself, but is 

 being continually produced by radioactive change. 



Lastly, the number of " rays " produced per second from 

 1 gram of a radio-element may be estimated. Since the 

 energy of each "ray" =10~ 5 ergs = 2*4 10 -13 gram-calories, 

 6 10 10 rays are projected every year from 1 gram of uranium. 

 This is approximately 2000 per second. The a. radiation of 

 1 milligram of uranium in one second is probably within 

 the range of detection by the electrical method. The methods 

 of experiment are therefore almost equal to the investigation 

 of a single atom disintegrating, whereas not less than 10 4 

 atoms of uranium could be detected by the balance. 



It has been pointed out that these estimates are concerned 

 with the energy of radiation, and not with the total energy 

 of radioactive change. The latter, in turn, can only be a 

 portion of the internal energy of the atom, for the internal 

 energy of the resulting products remains unknown. All 

 these considerations point to the conclusion that the 

 energy latent in the atom must be enormous compared 

 with that rendered free in ordinary chemical change. 

 Now the radio-elements differ in no way from the other 

 elements in their chemical and physical behaviour. On the 

 one hand they resemble chemically their inactive prototypes 

 in the periodic system very closely, and on the other thev 

 possess no common chemical characteristic which could be 

 associated with their radioactivity. Hence there is no reason 

 to assume that this enormous store of energy is possessed by 

 the radio-elements alone. It seems probable that atomic 

 energy in general is of a similar, high order of magnitude 

 although the absence of change prevents its existence being 

 manifested. The existence of this energy accounts for the 

 stability of the chemical elements as well as for the con- 



