PHYSICAL FORCES 95 



but a short piece of iron made red hot at one end cannot 

 be so held : iron being a much better conductor of heat 

 than charcoal. Wood, woollen substances, and fur, are 

 notoriously bad conductors of heat; while metals are 

 notoriously good conductors of it. Gold conducts much 

 more than twice as well as iron. If the conducting 

 power of gold be taken as 1000, iron is as 381, while 

 that of marble is but 23, and clay only n. 



We have before spoken * of the insufficiency of our 

 mere sensations as measures of temperature. If the 

 hand be plunged in water, really as warm as the air, the 

 water will feel colder. If also the hand be placed first on 

 fur, then on a wooden table, and then on a marble one, 

 the last will feel the coldest. This is because the marble 

 is a better conductor than the other substances, and so 

 conducts heat more quickly out of the warm hand. For 

 the same reason metal will feel still colder. It cannot 

 safely be touched in the Arctic regions with the naked 

 hand, because it conducts heat so rapidly from it as in 

 effect to burn it. If wood and metal be both made 

 equally hotter than the hand, the metal will feel much 

 the hotter of the two, because it will conduct heat into 

 the hand much more quickly. 



(2) Convection. In a solid body, heat passes through 

 it without occasioning any change of position between 

 its constituent parts so long as the particles cohere, 

 but in liquids it is quite otherwise. On account of 

 their extreme mobility some of the particles which 

 compose them are displaced by every change of tem- 

 perature the warmer particles ascending and so con- 

 veying the heat towards other parts of the liquid 

 mass, while by so doing they necessarily displace 



* See ante, p. 89. 



