CONDUCTION OF HEAT. 



27 



conducting power of the lead, which allows its surface to 

 be strongly heated by the brass. Professor Forbes finds that 

 the most intense vibrations are produced between the best con- 

 ductors and the worst conductors of heat,, the latter being the 

 cold bodies.* 



Our ordinary conceptions of the actual temperature of differ- 

 ent bodies, are much affected by the conducting powers of these 

 bodies. If we apply the hand, at the same time, to a good and to 

 a bad conductor, such as a metal and a piece of wood, which are 

 exactly of the same temperature by the thermometer, the good 

 conductor will feel colder or hotter than the other, from the 

 greater rapidity with which it conducts away heat from, or 

 communicates heat to our body, according as the temperature 

 of the metal and wood happens to be above or below that of 

 the hand applied to them. 



The diffusion of heat through liquids and gases is effected, in 

 a great measure, by the motion of their particles among each 

 other. When heat is applied to the lower part of a mass of 

 liquid the heated portions become lighter than the rest, and 

 ascend rapidly, conveying or carrying the heat through the 

 mass of the fluid. In a glass flask, for instance, containing 

 water, with which a small quantity of any light insoluble powder 

 has been mixed, a circulation of the fluid 

 may be observed upon the application of 

 the flame of a lamp to the bottom of the 

 vessel, the heated liquid rising in the 

 centre of the vessel, and afterwards 

 descending near its sides, as represented 

 in the annexed figure. But when heat 

 is applied to the surface of a liquid, this 

 circulation does not occur, and the heat 

 is propagated very imperfectly down- 

 wards. It has even been doubted whe- 

 ther liquids conduct heat downwards at 

 all, or indeed in any other way than by 

 conveying it, as above described. It can 

 be proved, however, that heat passes 

 downwards in fluid mercury, and hence 

 it is probable that all liquids possess a 

 slight conducting power similar to that of solids. 



* Edin. Phil. Trans, vol. xii. 



