506 EQUILIBRATION. 



still-continued tendency towards complete equilibrium, 

 through the still-continued dissipation of sensible motion, 

 or transformation of it into insensible motion. 



But there is another species of equilibration going on in 

 the Solar System, with which we are more nearly concerned 

 the equilibration of that molecular motion known as heat. 

 The tacit assumption hitherto current, that the Sun can con- 

 tinue to give off an undiminished amount of light and heat 

 through all future time, is fast being abandoned. Involv- 

 ing as it does, under a disguise, the conception of power pro- 

 duced out of nothing, it is of the same order as the belief 

 that misleads perpetual-motion schemers. The spreading 

 recognition of the truth that force is persistent, and that con- 

 sequently whatever force is manifested under one shape 

 must previously have existed under another shape, is carry- 

 ing with it a recognition of the truth that the force known 

 to us in solar radiations, is the changed form of some other 

 force of which the Sun is the seat ; and that by the gradual 

 dissipation of these radiations into space, this other force is 

 being slowly exhausted. The aggregative force by which 

 the Sun's substance is drawn to his centre of gravity, is the 

 only one which established physical laws warrant us in sus- 

 pecting to be the correlate of the forces thus emanating from 

 him: the only source of a known kind that can be assigned, 

 for the insensible motions constituting solar light and heat, 

 is the sensible motion which disappears during the progress- 

 ing concentration of the Sun's substance. We before saw it 

 to be a corollary from the nebular hypothesis, that there is 

 such a progressing concentration of the Sun's substance. 

 And here remains to be added the further corollary, that 

 just as in the case of the smaller members of the Solar Sys- 

 tem, the heat generated by concentration, long ago in great 

 part radiated into space, has left only a central residue 

 that now escapes but slowly; so in the case of that im- 

 mensely larger mass forming the Sun, the immensely 

 greater quantity of heat generated and still in process of 



