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SCIENCE 



[N. S. Vol. XXX. No. 765 



active elements are not immortal, they per- 

 ish after a life whose average value ranges 

 from thousands of millions of years in the 

 case of uranium to a second or so in the 

 case of the gaseous emanation from ac- 

 tinium. 



When the atoms pass from one state to 

 another they give out large stores of 

 energy, thus their descendants do not in- 

 herit the whole of their wealth of stored-up 

 energy, the estate becomes less and less 

 wealthy with each generation; we find, in 

 fact, that the politician, when he imposes 

 death duties, is but imitating a process 

 which has been going on for ages in the 

 case of these radioactive substances. 



Many points of interest arise when we 

 consider the rate at which the atoms of 

 radioactive substances disappear. Ruther- 

 ford has shown that whatever be the age of 

 these atoms, the percentage of atoms which 

 disappear in one second is always the same ; 

 another way of putting it is that the ex- 

 pectation of life of an atom is independent 

 of its age— that an atom of radium one 

 thousand years old is just as likely to live 

 for another thousand years as one just 

 sprung into existence. 



Now this would be the case if the death 

 of the atom were due to something from 

 outside which struck old and young indis- 

 criminately; in a battle, for example, the 

 chance of being shot is the same for old and 

 young; so that we are inclined at first to 

 look to something coming from outside as 

 the cause why an atom of radium, for ex- 

 ample, suddenly changes into an atom of 

 the emanation. But here we are met with the 

 difficulty that no changes in the external 

 conditions that we have as yet been able to 

 produce have had any effect on the life of the 

 atom ; as far as we know at present the life 

 of a radium atom is the same at the temper- 

 ature of a furnace as at that of liquid air- 

 it is not altered by surrounding the radium 



by thick screens of lead or other dense ma- 

 terials to ward off radiation from outside, 

 and what to my mind is especially signifi- 

 cant, it is the same when the radium is in 

 the most concentrated form, when its atoms 

 are exposed to the vigorous bombardment 

 from the rays given off by the neighboring 

 atoms, as when it is in the most dilute so- 

 lution, when the rays are absorbed by the 

 water which separates one atom from 

 another. This last result seems to me to 

 make it somewhat improbable that we shall 

 be able to split up the atoms of the non- 

 radioactive elements by exposing them to 

 the radiation from radium; if this radia- 

 tion is unable to affect the unstable radio- 

 active atoms, it is somewhat unlikely that it 

 will be able to affect the much more stable 

 non-radioactive elements. 



The evidence we have at present is 

 against a disturbance coming from outside 

 breaking up of the radioactive atoms, and 

 we must therefore look to some process of 

 decay in the atom itself; but if this is the 

 ease, how are we to reconcile it with the 

 fact that the expectation of life of an 

 atom does not diminish as the atom gets 

 older? We can do this if we suppose that 

 the atoms when they are first produced 

 have not all the same strength of constitu- 

 tion, that some are more robust than others, 

 perhaps because they contain more intrin- 

 sic energy to begin with, and will therefore 

 have a longer life. Now if when the atoms 

 are first produced there are some which will 

 live for one year, some for ten, some for a 

 thousand, and so on; and if lives of all 

 durations, from nothing to infinity, are 

 present in such proportion that the num- 

 ber of atoms which will live longer than a 

 certain number of years decrease in a con- 

 stant proportion for each additional year 

 of life, we can easily prove that the expec- 

 tation of life of an atom will be the same 

 whatever its age may be. On this view the 



