CONDUCTION. 813 



is a flow of heat until the whole is at the same tempera- 

 ture throughout. In the same manner heat flows also 

 from one body to another provided both are in contact 

 with each other. This kind of transmission of heat is 

 called conduction. Conduction takes place in different 

 bodies with very different velocities; hence bodies are 

 distinguished as c good conductors ' and ' bad con- 

 ductors ' of heat, a distinction which is very similar in 

 principle to that made in the conduction of electricity, 

 the more so as the best conductors of electricity are also 

 the best conductors of heat, and vice versa. The velocity 

 with which heat is transmitted even by the best con- 

 ductors is almost infinitely less than the velocity with 

 w r hich it is propagated by radiation. 



Metals conduct heat with much greater facility than 

 all other solids, but they differ among themselves very 

 considerably in their conducting power. Two wires of 

 the same length and thickness, viz., about 10 cm long and 

 l mm or 2 mm thick, one of iron and the other of copper, are 

 held at one end between the thumb and forefinger, one 

 wire in each hand, and the other ends of the wires are 

 placed in a flame. The copper wire soon becomes so 

 hot that it cannot be longer held in the hand, while the 

 iron wire may be held much longer. Copper is hence 

 a better conductor than iron, for in the copper wire 

 the heat flows with greater velocity from one end to the 

 other than in the iron wire. If the iron wire is thin it 

 can be held almost for any time ; the reason is that at 

 the outset a thin wire receives less heat than a thicker 

 one, and so much of the received heat is lost by radia- 

 tion and by contact with the air that the quantity which 

 actually flows from one end to the other is very little, 



