ATOMS AND SUNBEAMS. 133 



words, that the temperature teuds to fall. AVhat we have to discover 

 is the agent which at present prevents the solar temperature from 

 falling. We waut therefore to ascertain the means by which the molec- 

 ular velocities are preserved at the same average value, notwith- 

 standing that there is a constant tendency for these velocities to abate 

 in consequence of the losses of light and heat by radiation. We have 

 already explained how the gravitation of the sun constantly tends to 

 impart additional downward velocity to the molecules in its atmos- 

 phere. This is precisely the action which we now require. The con- 

 traction of the sun tends to an augmentation of the molecular velocities, 

 and this augmentation just goes to sujiply the loss of velocities which 

 is the conseipience of the radiation. A complete explanation of the 

 maintenance of the sun's heat is thus afforded. Observation no doubt 

 seems to show that the capacity for radiation is at present sensibly 

 constant, and this being so, we see that the gain of molecular velocities 

 from gravitation and their losses from radiation are at present just 

 adapted to neutralize each other. Nothing however that has as yet 

 been said demonstrates that the efticiency of the sun for radiating light 

 and heat must always be preserved exactly at its present value. 



It is quite i)ossible that if we had the means of studying the sun 

 heat for a hundred thousand years, we might find that the capacity 

 for radiation was slightly decreasing, or it may be that it would be 

 slightly increasing, for it is at least conceivable that the gain of 

 molecular velocity due to gravitation may, on the whole, exceed the 

 loss due to the dispersal of energy by radiation. On the other hand, 

 it is of course possible that the acquisition of velocity by gravitation, 

 though nearly sufficient to countervail the expenditure by radiation, 

 may not be quite enough, in which case the sun's temperature would be 

 slowly declining. 



It must not however be su])posed that the argument which we have 

 been here following attributes eternal vigor to the great luminary. It 

 will be noted that it is of the essence of the argument that the con- 

 traction is still in progress. If the contraction Avere to cease, then the 

 restitution of velocity by gravitation would cease also, and the s])eedy 

 dispersal of the existing heat by radiation would ]>resently produce 

 bankruptcy in the supply of sunbeams. Indeed, such bankruptcy 

 nuist arrive in due time, when, after certain millions of years, the sun 

 has so far contracted that it ceases to be a gaseous .mass. The vast 

 accumulated store of energy which is now being drawn upon to supply 

 the current radiation, will then yield such snjjplies no longer. Once 

 this state has been reached, a few thousand years more must witness 

 the extinction of the sun altogether as a source of light, and the great 

 orb, at present our splendid luminary, will then pass over into the 

 ranks of the innumerable Lost of bodies which were once suns, bnt are 

 now suns no longer. 



