Mechanics of Luminosity. 263 



Becquerel *, so that the coefficient of total emission of sodium 

 for all rays together is greater than 3' 2 X 10 3 . 



True Coefficient of Emission of 1 gr. Sodium. Comparison 

 with Platinum. Application of Kirchhojfs Law. 



28. From the data for the coefficient of total emission for 

 sodium, and the breadth of the sodium lines, we may obtain 

 the true coefficient of emission for the unit breadth in the 

 spectrum (cf. p. 161) for 1 gr. sodium. 



For this purpose we have only to divide the energy emitted 

 by the breadth of a sodium line; this according to diffraction 

 experiments (see below 32, p. 265) is £ of the distance be- 

 tween the centres of the two sodium lines — that is 015 (jljjl. 

 The true coefficient of emission is then, if we take 1 /jifM as unit 

 for the wave-length, ifiOO 



Sn * = <h? =10700; 



i. e. 1 gr. sodium in a region of the spectrum of the breadth 

 1 fjbfjb would emit per second 10700 g. cal., or in round num- 

 bers 1*1 x 10 4 if the brightness throughout this region were 

 alike. 



29. We may further compare the true coefficient of emission 

 of sodium vapour with that of solid platinum. We have 

 seen above (p. 259) that since the ordinate of the curve of energy 

 of platinum at the point which corresponds to the sodium 

 line is 11*35, the energy eradiated within the region of the 

 spectrum of breadth 1 /jl/jl by platinum is 



£E = 0-0 2 83x 11-35 x 0-001 x E = 9'4x 10- 5 E. 



Thus 1 gr. platinum emits (cf. p. 254) in this region 



9-4xl0- 5 x2-2xl0 4 =2'lcal. 



The ratio of the true coefficient of emission of sodium S Na 

 and platinum Sp t is therefore per gramme 



S Na 1-1 x 10* ■ 



SpT = -2^ = 5x1 °- 



The coefficient for sodium is thus much greater than that 

 for platinum. 



30. A film of platinum of 1 sq. centim. area and 10 ~ 5 

 centim. thickness, which contains 2 X 10~ 4 gr. platinum is 

 almost opaque. 



According to Kirchhoff's law a film of sodium vapour 

 which for equal area contains less substance in consequence 

 of its greater coefficient of emission for its particular rays, 



* Ed. Becquerel, Compt. Rend. xcix. p. 374 (1884). 



