38 Emission and Transmission of Heat 



MM' = R' (c i) and R' = M ~ M 'and as 



c i 



M=at (i + b() and M' = a't (i + b't} the value of R' would 

 be 



<: / c i 



Having thus obtained a general expression for the value of 

 R 1 ', that for A is easily deduced from it since A = M' R '. 



I employed the following method in obtaining the ratio c of 

 the radiations ; it depends on one of Dulong and Petit 's laws. Two 

 metallic vessels, one side of each being a vertical plane, bare or 

 covered with different materials, are placed with their plane faces 

 opposite and parallel, and equally distant from a thermopile con- 

 nected to a very sensitive galvanometer. One of the surfaces is 

 maintained at a constant temperature while the temperature of 

 the other is caused to vary until the effects produced on the ther- 

 mopile are the same, that is to say until the needle of the galvan- 

 ometer returns to zero. Designating by m and m' the radiat- 

 ing power of the two surfaces, by t and /' the excesses of their 

 temperatures above 0, that of the thermopile, we have for the 

 quantities of heat radiated according to Dulong and Petit ma 

 (a* i), and m'aQ (a 1 ' i), and as these quantities are equal, we 

 deduce that 



_. _ R _m a v i 



~tf~~~tn r = a'' i 



From all these experiments there result the following 

 formulas. 



789. The quantity of heat emitted by radiation to surround- 

 ings at a temperature differing but little from 12 C., and for 

 excesses of temperature between 25 C. and 65 C. is given by 

 the formula 



R=Kt(\ + .00560 ...... (a) 



A" is a coefficient depending on the form and the dimensions 

 of the body, / is the excess of temperature in degrees Centi- 

 grade. 



790. The quantity of heat lost by contact of air in the 

 same circumstances is given by the formula, 



A=fCt(i+ .00750 ..... (*) 



