132 ATOMS AND SUNBEAMS. 



body grown so cold as the earth draws its particles closer together. 

 The molecules in the solar i)liotosphere accordingly yield to a certain 

 extent to the gravitation which constantly seeks to draw them down. 

 The counteracting tendencies can not in the sun, as they do in the 

 earth, mask the direct and obvious eft'ect of gravitation. The conse- 

 quence is that the intense attraction which is capable of adding veloc- 

 ity to the molecules at the phenomenal rate of 457 feet per second, is i)er- 

 mitted to accom])lish something, and thus increase the average speeds 

 with which the molecules hurry along. To express the matter a little 

 more accurately, we should say that the downward velocity imparted 

 by gravitation, being compounded with the velocities otherwise pos- 

 sessed by the molecules, tends, on the whole, to increase the rate at 

 which they move. 



We shall now be able to discern what actually takes place as the sun 

 contracts by dis[)ersing heat, and in conse(pience of its decline in bulk 

 finds a store of energy liberated which it is permitted to use for the 

 pur])ose of sustaining its radiating capacity. Owing to the intense 

 heat which prevails in the photosphere, the molecules must there be in 

 very rapid movement; their nmtual encounters must be of the utmost 

 vehemence, and their internal vibrations, which are the consequences 

 of the shocks in the encounters, must be correspondingly energetic. It 

 is, as we have seen, these iuternal molecular vibrations which set the 

 ictlu'r in motion, and thus dispense solar heat ami light far and wide 

 through the universe. But this the molecules can onlv do at the 

 expense of the energy which they possess in virtue of their internal 

 vibrations. Unless therefore the internal molecular energy were to 

 be in some way recuperated from time to time, the radiating power 

 must necessarily flag. It is now plain that the necessary recuperation 

 takes places in the successive encounters. A molecule whose internal 

 energy of vibration is becoming exhausted by the effort of setting the 

 a*ther into vibration presently impinges against some other molecule, 

 and in consequence of the blow is again set into active vibration which 

 pernnts it to carry on the work of radiation anew, until its declining 

 energies have again to be sustained by some similar addition arising 

 from a fresh collision. Of course, we know that the internal molecular 

 energy thus acquired can not be created out of nothing. If the mole- 

 cule receives sucli accessions of internal energy, it must be at the 

 expense of the energy which is elsewhere. Obviously the only possible 

 source of such energy must be found in the movement of the molecule 

 as a whole, that is to say, in the velocity of translation with which it 

 rushes about among the other molecules. Thus we see that the imme- 

 diate effect of expenditure of heat or light by radiation is to diminish 

 the internal energies of the molecules. These energies are restored by 

 the transference of energy obtained from the general velocities of the 

 molecules regarded as moving projectiles. It follows that the velocities 

 of the several particles must on the whole tend to decline; in other 



