FIRE-CLAY AND ANTHRACITE. 313 



the next. Calling (for comparison sake) the conductivity 

 of silver 1000, that of copper is 736, gold 532, brass 236, 

 iron 119, marble and other building stones 6 to 12, porce- 

 lain 5, ordinary brick earth only 4, and fire-brick earth 

 less than this. Thus we may at once start upon our sub- 

 ject, with the practical fact that iron conducts heat thirty 

 times more readily than does fire-brick. 



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 portions 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. 



Radiation 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 sub- 

 stance 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 lamp- 

 black. 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 



