330 Prof. F. Rosetti's Experimental Researches 



in which q represents the quantity of heat given out by the 

 unit surface of the radiating l>ody in the unit of 

 time, 



t the temperature of the hot body, 



ti the surrounding temperature, 



a and b two constants. 



Although this formula corrrectly represents the phenomenon 

 of radiation in the experiments of Dulong and Petit, which 

 extended only between the limits of 0° and 280°, yet it has 

 been shown by Ericson that it is not applicable when the dif- 

 ference in temperature between the radiating body and the 

 surrounding temperature exceeds 80°. M. Jamin declares 

 that it is an empirical formula which becomes inexact at high 

 temperatures. Since, then, the formula of Dulong and Petit 

 is empirical and limited in its use, I have sought to substitute 

 for it another formula, which more correctly represents the 

 radiation as a function of the temperature of the radiating 

 body and the temperature of the medium surrounding the cold 

 body which becomes warm. After much consideration I have 

 decided to adopt the formula 



y=aT 2 (T-0)-b(T-0); 



in which y represents the thermal effect of the rays, measured 

 by the thermoelectric pile ; 



T is the absolute temperature of the radiating body; 



is the absolute temperature of the medium in 

 which the pile is placed ; 



a and b are two constants to be determined, which 

 depend on the nature of the thermoelectric instru- 

 ment, and which remain constant for one and the 

 same body radiating at all temperatures. 



The first of the two terms may be regarded as representing 

 the thermal effect produced by the rays in vacuo ; the second 

 represents the influence of the surrounding air. This formula 

 is, as regards its form, identical with that of Newton, since 

 the value of y is proportional to the difference T — 0; but 

 whilst in Newton's formula the emissive power of the radia- 

 ting body is considered to be independent of the temperature, 

 in the formula proposed by myself the emissive power is re- 

 presented by ET 2 ; that is, it is proportional to the square of 

 the absolute temperature of the radiating body. In a body 

 with a maximum emissive power, such as lampblack, E should 

 be equal to 1 only for T=l ; but as T increases the emissive 

 power should also increase, in proportion to the square of the 

 temperature. This supposition was confirmed by many of my 



