38 SCIENCE IN SHORT CHAPTERS. 



brass 236, iron 119, marble and other building stones 6 to 12, 

 porcelain 5, ordinary brick earth only 4, anid fire-brick earth 

 less than this. Thus we may at once start upon our subject, 

 with the practical fact that iron conducts heat thirty times 

 more readily than does fire-brick. 



j Convection is different from conduction, inasmuch as it is 

 \ effected by the movements of the something which has been 

 heated by contact with something else. Water is a very bad 

 conductor of heat, much worse than fire-brick, and yet, as we 

 all know, heat is freely transmitted by it, as when we boil water 

 in a kettle. If, however, we placed the water in a fire-clay 

 kettle, and applied the heat at the top we should have to wait 

 for our tea until to-morrow or the next day. When the heat is 

 applied below, the hot metal of the kettle heats the bottom film 

 of water by direct contact ; this film expands, and thus, being 

 lighter, rises through the rest of the water, heating other por- 

 tions by contact as it meets them, and so on throughout. 

 The heat is thus conveyed, and the term " convection" is 

 based on the view that each particle is a carrier of heat as it 

 proceeds. Air conveys heat in the same manner ; so may all 

 gases and liquids, but no such convection is possible in solids. 

 The common notion that " heat ascends " is based on the 

 well-known facts of convection. It is the heated gas or liquid 

 that really ascends. No such preference is given to an upward 

 direction, when heat is conducted or radiated. 



JKadiation is a flinging off of heat in all directions by the 

 heated body. Radiation from solids is mainly superficial, and 

 it depends on the nature of the heated surface. The rougher 

 and the more porous the surface of a given substance the 

 better it radiates. Bright metals are the worst radiators ; 

 lampblack the best, and fire-brick nearly equal to it. To show 

 the effect of surface, take three tin canisters of equal size, one 

 bright outside, the second scratched and roughened, the third 

 painted over with a thin coat of lampblack. Fill each with hot 

 water of the same temperature, and leave them equally 

 exposed. Their rates of radiation will then be measurable by 

 their rates of cooling. The black will cool the most rapidly, 

 the rough canister next, and the bright one the slowest. 



Radiant heat may be reflected like light from bright surfaces, 

 the reflecting substance itself becoming heated in a proportion 

 which diminishes just as its reflecting powers increase. Good 

 reflectors are bad radiators and bad absorbers of heat, and the 

 power of absorbing heat, or becoming superficially hot when 



