22fi MODERN VIEWS ON MATTER. 



more t!i;iii one in a million million. Nevertheless, just as oecasional 

 thouuli rare encounters take place in the heavens, followed by the 

 l»]aze of a new and temporary stai", so, though probal)ly not l)v the 

 same mechanism, liere and there a few out of the billions of atoms in 

 any perceptilile speck of radium arrive in due time at the unstal^le con- 

 dition 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 knowMi. being able to pass through an inch of solid 

 iron in perceptible quantity. 



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

 field, then, is that a very small numl)er — an almost infinitesimal pro- 

 portion — of the atoms are constantly breaking up. throwing away a 

 small portion, say 1 per cent, of themselves with innnense violence 

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

 slightly diti'erent substance, which, however, is still extremely unstable, 

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

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

 in tlie unstable condition,. and so giving rise to fresh and fresh products 

 of its oAvn decay, till a comparativel}' stable state is reached, or till the 

 process passes beyond our means of detection. 



Roughly, the process may l)e likened in some respects to the con- 

 densation or contraction of a ne])ula. The particles constituting a 

 whirling nebula fall togetlier until the centrifugal fc^rceof the periph- 

 eral portions exceeds the gravitative pull of the central mass, and then 

 they are shrunk off and left ludiind, afterwards agglomerating into a 

 planet, wliile the residue goes on shrinking and evolving fresh bodies 

 and generating heat. A nebvila is not hot, l)ut it has an immense store 

 of potential energy, some of which it can turn into heat, and so form 

 a hot central nucleus or sun. A radium atom is not hot, but it, too, 

 has a great store of potential energy, innnense in proportion to its 

 mass, for it is controlled by electrical, not by gravitational forces; and 

 just as the falling together of the solar material generates heat, so 

 that a shrinkage of a few yards per century can account for all its 

 tremendous emission, so it has been calculated that the collapsing of 

 the electrical constituents of a radium atom, by so little as 1 per 

 cent of their distance apart, can supply the whole of the energy of 

 the observed radiation — large though that is — for something like 

 thirty thousand years. 



15. It does not follow that the life of a piece of radimn is as great 

 as that; the data are uncertain at present, but there is absolutely no 

 ground for the popular and gratuitous surmise that it emits energy 

 without loss or waste of any kind, and that it is competent to go on 

 forever. The idea, atone time ii-responsibly mooted, that it contra- 



