MODERN VIEWS ON MATTER. 301 



of its own decay, till a comparatively stable state is reached, or till the 

 process passes beyond our means of detection. 



Eoughly, the process may be likened in some respects to the con- 

 densation or contraction of a nebula. The particles constituting a 

 whirling nebula fall together until the centrifugal force of the periph- 

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

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

 planet; while the residue goes on shrinking and evolving fresh bodies 

 and generating heat. A nebula is not hot, but 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, immense 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 one per 

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

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

 30,000 years. 



15. It does not follow that the life of a piece of radium 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 

 for ever. The idea, at one time irresponsibly mooted, that it contra- 

 dicted the principle of the conservation of energy, and was troubling 

 physicists with the idea that they must overhaul their theories — a 

 thing which they ought always to be delighted to do on good evidence 

 — this idea was a gratuitous absurdity and never had the slightest 

 foundation ; but the notion that radium was perhaps able to draw upon 

 some unknown source or store of energy, without itself suffering loss, 

 was a possibility which has not yet wholly disappeared from some minds. 

 Sir W. Crookes, for instance, suggested that it might somehow utilize 

 the most quickly moving atoms of air, after the fashion of a Maxwell 

 demon — a possibility that should always be borne in mind as a con- 

 ceivable explanation of the power of some living organisms. It is 

 much more reasonable to suppose, however, that radium and the other 

 like substances are drawing upon their own stores of internal atomic 

 energy, and thereby gradually disintegrating and falling into other, 

 and ultimately into more stable, forms of matter. 



Not that it is to be supposed that even these are finally and abso- 

 lutely stable: these too are subject to radiation loss, and so must be 

 liable to decay; but at a vastly slower rate, perhaps not more than a 

 few hundred atoms changing and diffusing away each second — a process 



