1887.] on the Suiis Heat. 9 



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 uttermost 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. 5). I called it so because Davy first used the 

 expression " repulsive motion " to describe the fine intermolecular 

 motions to which he and other founders of the Kinetic Theory of 

 Heat attributed the elastic resistance to compression presented by 

 gases and fluids. 



Imagine, instead of the atoms and molecules of the various 

 substances which constitute the sun's mass, a vast number of elastic 

 globes, like schoolboys' marbles or billiard balls. Consider first, 

 anywhere on our earth a few million such balls put into a room, 

 large enough to hold a thousand times their number, with perfectly 

 hard walls and ceiling, but with a real wooden floor ; or, what would 

 be still more convenient for our purpose, a floor of thin elastic sheet 

 steel, supported by joists close enough together to prevent it from 

 drooping inconveniently in any part. Suppose in the beginning the 

 marbles to be lying motionless on the floor. In this condition they 

 represent the atoms of a gas, as for instance, oxygen, nitrogen, or 

 hydrogen, absolutely deprived of heat, and therefore lying frozen, 

 or as molecular dust strewn on the floor of the containing vessel. 



If now a lamp be applied below the oxygen, nitrogen, or hydrogen, 

 the substance becoming warmed by heat conducted through the floor, 

 will rise from its condition of absolutely cold solid, or of incoherent 

 molecular dust, and will spread as a gas through the whole enclosed 

 space. If more and more heat be applied by the lamp the pressure 

 of the gas outwards in all directions against the inside of the 

 enclosing vessel will become greater and greater. 



As a rude mechanical analogue to this warming of a gas by heat 

 conducted through the floor of its containing vessel, from a lamp 

 held below it, return to our room with floor strewTi with marbles, 

 and employ workmen to go below the floor and strike its underside 

 in a great many places vehemently with mallets. The marbles in 

 immediate contact with the floor will begin to jump from it and fall 

 sharply back again (like water in a pot on a fire simmering before it 

 boils). If the w^orkmen work energetically enough there will be 

 more and more of commotion in the heap, till every one of the balls 

 gets into a state of irregular vibration, up and down, or obliquely, or 

 horizontally, but in no fixed direction ; and by mutual jostling the 

 heap swells up till the ceiling of the room prevents it from swelling 

 any further. Suppose now the floor to become, like the walls and 



