314 SCIENCE IN SHORT CHAPTERS. 



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 ab- 

 sorbers of heat, and the power of absorbing heat, or becom- 

 ing superficially hot when exposed to radiant heat, is exactly 

 proportionate to radiating efficiency. 



Fire-clay is a good absorber of radiant heat, i.e., it becomes 

 readily heated when near to hot coals or flames, without 

 requiring actual contact with them. It is an equally good 

 radiator. 



Let us now apply these facts to fire-clay in fireplaces, 

 beginning with ordinary open grates used for the warming 

 of apartments ; first supposing that we have an ordinary 

 old-fashioned grate all made of iron front, sides, and back, 

 as well as bars, and next that we have another of similar 

 form and position, but all the fire-box and the back and 

 cheeks of the grate made of fire-clay. 



It is evident that the fire-clay not in actual contact with 

 the coals, but near to them, will absorb more heat than the 

 iron, and thus become hotter. Even at the same temperature 

 it will, radiate much more heat than iron, but being so much 

 hotter this advantage will be proportionately increased. An 

 open fireplace lined throughout with fire-clay thus throws 

 into the room a considerable amount of its own radiation 

 in addition to that thrown out from the coal. 



But what becomes of this portion of the heat When the 

 fireplace is all of metal? It is carried up the chimney by 

 convection, for the metal, while it parts with less heat by 

 radiation, gives up more to the air by direct contact. There- 

 fore, if wo must burn our coals inside the chimney, we lose 

 less by burning them in a fire-clay box than in a metal 

 box. 



Count Rumford demonstrates this, and described the 

 best form of open firegrate that can be placed in an ordinary 

 English hole-in-the-wall fireplace. The first thing to be 

 done, according to his instructions, is to brick up your large 

 square fireplace recess, so that the back of it shall come 



