COMMUNICATION OF HEAT. 



217 



Fig. 198. 





mer are sucli as allow heat to pass freely through them ; 

 the latter comprise those which do not give au easy pass- 

 age to it. 



Dense solid bodies, like the metals, are the best con- 

 ductors of heat ;* light, porous substances, more esi^ecially 

 those of a fibrous nature, are the worst conductors of heat. 



The different conducting power of various solid substances may be strik* 

 icgly shown by taking a series of rods of equal length and thickness, coating 

 one of their extremities with wax, and placing the other extremities equally, 

 in a source of heat The wax will be found to entirely melt off from some 

 of the rods before it has hardly softened upon others. 

 What is the 495. Liquids are almost 

 tower"""/ uq- absolutc non-conductois of 

 •""i^- heat. 



If a small quantity of alcohol be poured on the sur- 

 face of water and inflamed, it will continue to bum 

 for some time. (See Fig. 198.) A thermometer, 

 immersed at a small depth below the common sur- 

 face of the spirit and the water, will fMl to show any 

 increase in temperature. 



Another and more simple experiment proves the 

 Game fiict ; as when a blacksmith immerses his red- 

 hot iron in a tank of water, the water which sur- 

 rounds the iron is made boding hot, whde the water 

 not immediately in contact with it remains quite cold. 



Fig. 199. If a tube nearly filled with water is held 



over a spirit lamp, as in Fig. 199, in such a 

 manner as to direct the flame against the 

 upper layers of the water, the water will be 

 observed to boil at the top, but remain cool 

 below. If quicksilver, on the contrary, be 

 so treated, its lower layers ^ill speedily be- 

 come heated. The particles of mercury will communicate the heat to each 

 Jther, but the particles of water will not do so. 



A stone, or marble hearth in an apartment, feels colder to 



btone, or°mar" the feet than a woolen carpet, or hearth-rug, not because tho 



ble hearth feel Q^e is hotter than the other, for both are really of the sa;; 3 



but because the stone and marble are good 



colder than a 



carpet? temperature, 



• The following table exhibits the relative conducting power of different substances, 

 ihe ratio expressing the conducting power of gold being taken at 100 ; 



10 



