ON THE SUN'S HEAT. 393 



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 inclosed 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 strewn 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 workmen 

 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, 



