ON THE SUN'S HEAT. 



391 



of realisation, could direct the work done by 

 mutual gravitation between all the parts of the 

 shrinking mass, to actually generate its heat- 

 equivalent in an ocean of white-hot liquid 

 covering the sun's surface, and so keep it white- 

 hot while constantly radiating out heat at the 

 actual rate of the sun's heat-giving activity. Let 

 us now consider a little more in detail the 

 real forces and movements actually concerned 

 in the process of cooling by radiation from the 

 outermost region of the sun, the falling inwards 

 of the fluid thus cooled, the consequent mixing 

 up of the whole mass of the sun, the resulting 

 diminished elastic resistance to pressure in equi- 

 dense parts, and the consequent shrinkage of 

 the whole mass under the influence of mutual 

 gravitation. I must first explain that this 

 "elastic resistance to pressure" is due to heat, 

 and is, in fact, what I have, in the present 

 lecture, called "Sir Humphry Davy's repulsive 

 motion" (p. 381). I called it so because Davy 

 first used the expression " repulsive motion " 

 to describe the fine intermolecular motions to 



