Mechanics of Luminosity. 255 



required to raise the quantity of platinum in question from 

 0° C. to 1000°. 



The mean specific heat of platinum between 0° and f°, 

 according to Violle*, is 



c *= 0*0317 + 0'0 5 6t; 

 and therefore between 0° and 1000° C, 



c ; 000 = 0-038. 



To heat one gramme from 0° 0. to 1000° C. we therefore 

 require 1000x0*038 = 38 calories; and to heat the surface- 

 layer, 



2 x 10~ 4 x 38 cal. =7-6 x 10~ 3 cal. 



The quantity of energy radiated per second at 1000° is 

 therefore about 600 times greater than the quantity commu- 

 nicated in heating from 0° to 1000°. 



If, further, we have a platinum wire of r centim. radius 

 and 1 centim. length, then at 1000° the quantity of energy 

 lost per second by radiation M, and the quantity W commu- 

 nicated in heating from 0° to 1000°, are given by the formula} 



M = 2irr x 4'7, W = tjt 2 x 21*5 x 38 ; 

 therefore W/M=87xr. 



We see from this that, with a wire about ^ centim. in 

 thickness, the energy radiated in a second and that commu- 

 nicated in heating from 0° are nearly equal. With thinner 

 wires the latter diminishes very rapidly in comparison with 

 the former, 



Exactly similar considerations of course apply to the case 

 of glowing and radiating platinum foil. 



22. The method employed for the determination of the 

 radiation gives it, in the first place, according to order of 

 magnitude. The numbers just quoted show, in fact, how 

 extraordinarily great the radiation is. The surface-layer must 

 therefore cool rapidly. The loss of energy thus caused is 

 instantly supplied by conduction from the interior hot. por- 

 tions at the expense of the work done by the current. Since 

 the outer portions are, in any case, cooler than the inner, 

 their resistance must be less. But the resistance measured is 

 a mean of the various concentric layers. Therefore it is not 

 at once possible, without a thorough inquiry into the relation- 

 ships of conductivity for heat &c, to obtain a reliable conclu- 

 sion, from the observed resistances, as to the actual tempera- 

 ture of the radiating surface t« 



* Comptes Rendus, lxxxv. p. 543 (1877). 



t Compare also (amongst others) the work of G. Basso, Natura, iii. 

 pp. 226, 304. 



U2 



