312 SCIENCE IN SHORT CHAPTERS. 



FIRE-CLAY AND ANTHRACITE. 



FOR household fire-places, whether open or closed, these 

 may be regarded as the material and the fuel of the future, 

 and should be more generally and better understood than 

 they are. 



The merits of fire-clay were fully appreciated and de- 

 scribed nearly a hundred years ago by that very remarkable 

 man, Benjamin Thompson, Count of Rumford. Any sound 

 scientific exposition of the relative value of fire-clay and 

 iron as fire-place materials can be little more or less than a 

 repetition of what he struggled to teach at the beginning 

 of the present century. 



It is impossible to fairly understand this subject unless 

 we start with a firm grasp of first principles. The business 

 before us is to get as much heat as possible from fuel burn- 

 ing in a certain fashion, and to do this with the smallest 

 possible emission of smoke. 



Substances that are hotter than their surroundings com- 

 municate their excess of temperature in three different 

 ways; 1st, by Conduction; 2d, by Convection; 3d, by Radia- 

 tion. All of these are operating in every form of fire-place, 

 but in very different proportions according to certain vari- 

 ations of construction. 



To demonstrate the conduction of heat, hold one end of 

 a pin between the finger and thumb, and the other end in 

 the flame of a candle. The experiment will terminate 

 very speedily. Then take a piece of a lucifer match of the 

 same length as the pin, and hold that in the candle. This 

 may become red-hot and flaming without burning the 

 fingers, as the pin did at a much lower temperature. It 

 matters not whether the pin be held upwards, downwards, 

 or sideways, the heat will travel throughout its substance, 

 and this sort of traveling is called "conduction," and the 

 pin a " conductor" of heat. The conducting power of dif- 

 ferent substances varies greatly, as the above experiment 

 shows. Metals generally are the best conductors, but they 

 differ among themselves; silver is the best of all, copper 



