300 POPULAR SCIENCE MONTHLY. 



It represents a fact previously wanted on theoretical grounds. For 

 how is radio-activity to be explained ? It looks as if the massive and 

 extremely complex atoms of a radio-active substance were liable to get 

 into an unstable condition, probably reaching this condition whenever 

 any part of it attempts, or is urged, to move with the velocity of light. 

 I have shown elsewhere* that the mere fact of radiation will act as a 

 resisting medium and increase the speed of the particles automatically, 

 on the same principle that a comet would be accelerated if it met with 

 resistance; since the inverse square law applies to electrical central 

 forces. Electrical mass is not strictly constant: it is a function of 

 speed, but in such a way that it is practically constant until the velocity 

 of light is very nearly attained. That is a critical velocity, which 

 apparently can not be surpassed. When this critical speed is reached, 

 any electrified body becomes suddenly of infinite mass, and something 

 is bound to happen. What that something is, it is not easy theoret- 

 ically to say; but the partial or incipient disintegration or dissociation 

 of the atom, and the flying away of a portion with a speed comparable 

 to that of light, is no unlikely result. 



Out of the whole multitude of atoms, even of the atoms of a con- 

 spicuously radio-active substance, it is probable that only a very few 

 get into this unstable or critical condition at any one time; perhaps 

 not more than one in a million million ; nevertheless, just as occasional 

 though rare encounters take place in the heavens, followed by the blaze 

 of a new and temporary star, so, though probably not by the same 

 mechanism, here and there a few out of the billions of atoms in any 

 perceptible speck of radium arrive in due time at the unstable condi- 

 tion, and break down into something else, with energetic radio- 

 activity during the sudden collapsing process; emitting in the process 

 of collapse not only the main projected substance, but likewise also a 

 few electrons and those X-rays which always accompany a sudden 

 electric jerk or recoil. And the X-rays so emitted are of the most 

 penetrating kind known, being able to pass through an inch of solid 

 iron in perceptible quantity. 



14. The hypothesis concerning radio-activity which is now in the 

 field, then, is that a very small number, an almost infinitesimal pro- 

 portion, of the atoms are constantly breaking up; throwing away a 

 small portion, say one per cent, of themselves, with immense violence, 

 at about one tenth of the speed of light; the remainder constitute a 

 slightly different substance, which, however, is still extremely unstable, 

 and therefore radio-active, going through its stages with much greater 

 rapidity than the radium itself, because practically the whole of it is 

 in the unstable condition, and so giving rise to fresh and fresh products 



* See Nature for June 11, 1903. 



