Chap, xxxv.] CONDUCTION OF HEAT. 457 



transference of movement. The particles of the part 

 of the rod that is in the fire are thrown into vibra- 

 tion more or less vigorous, according to the heat of 

 the fire, and the vibration of these particles is com- 

 municated to others in their neighbourhood, till all 

 the particles of the bar are thrown into oscillation. 

 All substances do not conduct heat with the same 

 facility. Copper is a specially good conductor of 

 heat ; iron is not such a good conductor ; while stone, 

 glass, and organic substances, wood, hair, feathers, 

 etc., are bad conductors. Liquids and gases have 

 feeble conducting power. The difference in the con- 

 ducting power of different substances is measured by 

 the quantity of heat that will pass in a unit of time 

 through a unit area of the substance of unit thickness 

 with 1 difference of temperature between the sur- 

 faces. The number obtained is the " coefficient of 

 conductivity." 



Wood and brick have a less conducting power 

 than stone ; they are, therefore, best for house build- 

 ing, where it is desired to conduct heat either out- 

 w^ards or inwards as little as possible. According to 

 Rumford, the fur of the hare conducts least of all 

 substances used for clothing. Following it conies 

 eider-down, silk, wool, cotton, and hemp. The prac- 

 tical application of these facts is apparent. What is 

 called warm clothing is so because it prevents the 

 outward passage of the heat of the body, at least by 

 conduction, and the warmth is thus retained. Thus the 

 reason of the preference for woollen clothing in cold 

 weather is apparent. But woollen clothing will not 

 only prevent the passage of warmth outward from 

 the body, it will also prevent the passage of heat 

 inwards to the body. An example of this is seen 

 when a block of ice is surrounded by woollen material 

 to prevent it thawing. It may be packed in sawdust 

 for the same purpose. The heat is not conducted 



