CONDUCTION OF HEAT. 31 



other hand, the conductivity of metals is high, as we may 

 learn by placing our hand on the middle part of a poker 

 which has one end red hot. 



Knowledge of the conductive powers of substances can be 

 utilised in various practical ways. Thus, by enveloping a 

 piece of ice in several folds of a blanket or woollen rug, we 

 prevent the ice to a great extent from melting, even during a 

 summer's day ; because the protecting material is unable to 

 conduct from the outside a sufficiency of heat to melt the ice. 

 An instance of obstruction to the outward transmission of 

 heat is afforded by the " Norwegian stove," which consists of 

 a box lined with thick felt, and which is used to keep food 

 hot for several hours. It loses its heat so slowly that if a pot 

 containing the raw materials of a stew or soup is brought to 

 boiling point and then put into this box, the process of cook- 

 ing will proceed without interruption, and will become com- 

 pleted in a few hours with only a small loss of temperature. 



The transmission of heat is similar to that of light and 

 sound, in that its intensity varies inversely as the square of 

 the distance travelled. Thus, the light of a candle or the 

 sound of a note of music is four times as strong at a distance 

 of i yard as it is at a distance of 2 yards. Consequently, we 

 may infer that a rug I in. thick would be four times as warm 

 as a rug half an inch thick, both being of the same material. 



The nature of the sensation of heat or cold produced by 

 contact is greatly dependent on the conductivity of the body 

 that is touched. For instance, among substances of the same 

 temperature, a bad conductor of heat, such as wool or wood, 

 will feel warmer to the touch than a better conductor, such as 

 iron or stone ; a fact which accounts for the not uncommon 

 idea of persons unacquainted with physics, that the former 

 class of objects are habitually of a higher temperature than 

 the latter, under similar conditions of heat. 



The following table of the comparative heat-conducting 



