FIRE-CLAY AND ANTHRACITE. 39 



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 requirs 

 ing actual contact with them. It is an equally good radiator. 



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

 ning with ordinary* open grates used for the warming of apart- 

 ments ; 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 fire- 

 place is all of metal ? It is carried up the chimney by con- 

 vection, for the metal, while it parts with less heat by radia- 

 tion, gives up more to the air by direct contact. Therefore. 

 if we 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 Eng- 

 lish 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 forward to 

 about 4 in. from the front inside face of the chimney, thus 

 contracting the throat of the chimney, just behind the mantel, 

 to this small depth (Rumford' s device for sweeping need not 

 be here described). The sides or " covings" of this shal- 

 lowed recess are now to be sloped inward so that each one 

 shall horizontally be at an angle of 135 degrees to the plane 

 of this new back, and meet it at a distance of six or more inches 

 apart, according to the size of grate required. The covings 

 will thus spread out at right angles with each other, and leave 

 an annular opening to be lined with fire-brick and run straight 

 up to the chimney. The fire-bars and grate bottom to be sim- 

 ply let into this as far forward as possible. 



