and the Economy of Fuel. 41 



It is not, however, burning bodies alone that emit 

 calorific rays. All bodies those which are fixed and 

 incombustible as well as those which are inflammable, 

 fluids as well as solids are found to throw off these 

 rays in great abundance, as soon as they are heated to 

 that degree which is necessary to their becoming lumi- 

 nous in the dark, or till they are red-hot. 



Bodies even which are heated to a less degree than 

 that which is necessary to their emitting visible light 

 send off calorific rays in all directions. This is a mat- 

 ter of fact, which has been proved by experiment. Do 

 all bodies, at all temperatures, freezing mercury as 

 well as melting iron, continually emit these rays in 

 greater or less quantities, or with greater or less veloci- 

 ties? Are bodies cooled in consequence of their 

 emitting these rays? Do these calorific rays always 

 generate heat, even when the body by which they are 

 stopped or absorbed is hotter than that from which the 

 rays proceeded ? But I forget that I promised not to 

 involve myself in abstruse speculation. To return, 

 then. Whatever may be the nature of the rays 

 emitted by burning fuel, as one of their known proper- 

 ties is to generate heat, they ought certainly to be very 

 particularly attended to in every arrangement in which 

 the economy of heat, or of fuel, is a principal object in 

 view. 



As these calorific rays generate heat in the body by 

 which they are stopped or absorbed, and not in the me- 

 dium through which they pass, it is necessary to dispose 

 those bodies which are designed for stopping them in 

 such a manner that they may easily and necessarily 

 communicate the heat they thus acquire to the body 

 upon which it is intended that it should operate. 



