96 ELEMENTS OF SCIENCE 



colder particles which, being heavier, take the places of 

 those which have been expanded by heat, and have so 

 become specifically lighter. 



In aeriform bodies, convection also takes place; any 

 hot body causing upward currents at once. This is the 

 cause of that apparent twinkling movement of objects 

 before spoken of * as often to be observed, on a brilliant 

 hot day, immediately above the surface of the ground. 

 It is produced by waves of air of different temperatures, 

 and therefore of different densities, which refract f the 

 rays of light, and so modify the appearance of objects 

 seen through such waves of air. 



The greater the power of convection a body possesses, 

 the less its power of conduction. Thus water in a 

 closed tube with a piece of ice at the bottom of it 

 may be made to boil at the surface while the ice will 

 remain unmelted. Aeriform bodies are even worse 

 conductors than fluids are. 



(3) Radiation. It is a most familiar fact that we 

 can very quickly obtain much warmth by standing in 

 front of a bright open fire or a mass of red-hot embers. 

 This heat is certainly not obtained by conduction, 

 seeing how bad a conductor of heat air is. It is also not 

 due to convection, for the effect is too instantaneous and 

 the hot air (displaced by the rush of cold air towards the 

 bright and glowing mass) would not be conveyed horizon- 

 tally outwards, but upwards to the space whence the 

 cold air had descended. 



The fact is that the incandescent or glowing mass 

 gives forth what are called rays of heat. These " rays " 

 may be supposed to be (a) an influence, or (b) a mode 

 of motion, or (c) a substance emitted by the fire and 



See ante, p. 83. t See post, pp. 104 and 105. 



