CONDUCTION AND RADIATION. 



offer some difficulty. It appeared that cold was reflected no less thar 

 heat. A mass of ice, when its effect was concentrated on a thermo- 

 meter by a system of mirrors, made the thermometer fall, just as a 

 vessel of hot water placed in a similar situation made it rise. Was 

 cold, then, to be supposed a real substance, no less than heat ? 



The solution of this and similar difficulties was given by Pierre 

 Prevost, professor at Geneva, whose theory of radiant heat was pro- 

 posed about 1790. According to this theory, heat, or caloric, is con- 

 stantly radiating from every point of the surface of all bodies in 

 straight lines ; and it radiates the more copiously, the greater is the 

 quantity of heat which the body contains. Hence a constant ex- 

 change of heat is going on among neighboring bodies ; and a body 

 grows hotter or colder, according as it receives more caloric than it 

 emits, or the contrary. And thus a body is cooled by rectilinear rays 

 from a cold body, because along these paths it sends rays of heat in 

 greater abundance than those which return the same way. This the- 

 ory of exchanges is simple and satisfactory, and was soon generally 

 adopted ; but we must consider it rather as the simplest mode of ex- 

 pressing the dependence of the communication of heat on the excess 

 of temperature, than as a proposition of which the physical truth is 

 clearly established. 



A number of curious researches on the effect of the different kinds 

 of surface of the heating and of the heated body, were made by 

 Leslie and others. On these I shall not dwell ; only observing that 

 the relative amount of this radiative and receptive energy may be ex- 

 pressed by numbers, for each kind of surface ; and that we shall have 

 occasion to speak of it under the term exterior conductivity ; it is 

 thus distinguished from interior conductivity, which is the relative 

 rate ?,t which heat is conducted in the interior of bodies. 6 



Sect. 3. Verifications of the Doctrines of Conduction and Radiation. 



THE interior and exterior conductivity of bodies are numbers, which 

 enter as elements, or coefficients, into the mathematical calculations 

 founded on the doctrines of conduction and radiation. These cocffi- 



6 The terra employed by Fourier, conductibil'dy or conduciUHty, suggests ex- 

 pressions altogether absurd, o,s if the bodies could be called conductlble, or con 

 ductile, with respect to heat : I have therefore ventured upon a slight altera- 

 tion of the -word, and have used the abstract term which analogy would 

 suggest, if we suppose bodies to be conductive in this respect. 



