162 On the Mechanics of Luminosity. 



energy emitted is 



S A = 1 s K d\. 



J Ki 



The quantity S A we will designate as the total coefficient of 

 emission of the unit weight between the wave-lengths \ Y and \ 2 . 

 It is the energy emitted in the unit time by the unit weight 

 of the body in question corresponding to all rays between the 

 wave-lengths X x and X 2 . The two quantities S A and S\ are 

 exactly analogous to the total quantity of heat necessary to 

 heat a body from t x ° to t 2 ° and the true specific heat. The two 

 quantities Sa and s A in the form given above have not been 

 experimentally determined as yet. We have above all not 

 referred the emission to a definite quantity of the radiating 

 body, but only to the unit of surface of the particular body. 

 The molecular coefficients of emission are obtained by multi- 

 plying S\ and Sa by the molecular weight of the body under 

 investigation. 



15. In this investigation of spectra two problems may 

 occur. We determine 



(1) The total coefficient of emission S Ai a 2 between the wave- 

 lengths X x and X 2 °f a body which is maintained in a constant 

 condition (e.g. of a platinum wire of constant temperature). 

 Then the total coefficient of emission can be determined for 

 the whole spectrum from X = to \=oo, or for particular 

 parts of it, which may ultimately consist of one or more so- 

 called spectral lines or bands stretching continuously between 

 every two wave-lengths. Then S assumes the value 



S o = | s k d\ and 8 = I s^dX + j s k dX + . . . 



Jo J A, Ja 3 



It is to be observed that the value of the first integral cannot 

 be directly determined, since we do not know what the radia- 

 tion is either for very small wave-lengths or for very great 

 wave-lengths, but our experiments are limited to a very small 

 portion of the possible rays. Further, we must observe that 

 in our experiments as soon as s x relates to rays which are 

 also given off by surrounding bodies, we determine not the 

 coefficient itself but s K — a k , where a K denotes the coefficient of 

 emission of the bodies serving for the measurement of the 

 wave-length X, also in calorimetric measure. 



(2) We determine the true coefficient of emission s k for a 

 single wave-length if belonging to a definite point of the 

 spectrum. Here we must observe that line-like portions of 

 the spectrum are not directly comparable with continuous 



