( 4ir> ) 



uiuloiibtcdly he coiisidoi'cd as (lelerinined by tlic s(;x(o of the inaUer 

 contained within the element; for this reason an electromotive force 

 equivalent to those processes can only depend on quantities deter- 

 mined by that state; it cannot be altered by changing the state of 

 the system outside the element considered, or the form and magnitude 

 of the whole body. The formula (40), which indeed is determined 

 by the state of things within the element s, must therefore be applied 

 to an element of volume of all ponderable matter. It will be clear 

 also that we have to add the following formnlae for the amplitudes 

 of the electromotive forces in the directions of y and z, 



As to the phases of the three electromotive forces, we shall suppose 

 them not only to change irregularly from one element to another, 

 but also to be mutually independent in one and the same element, 

 so that the phase-differences between the thi'ee forces have very 

 different values in neighbouring infinitely small spaces. In virtue of 

 this assumption the intensities of the radiation due to the different 

 causes may be added to each other. 



Till now we have only accounted for the flow of energy (36), a 

 part of the total flow represented by (35). We shall show in the 

 next paragraph that the remaining part 



^ • (4->) 



cr 



is precisely the radiation brought about by the electromotive forces 

 we have supposed to exist in the direction of OF, and that the forces 

 acting in that ot OZ cannot give rise to a radiation across the 

 element a'. After having proved these propositions, we may be sure 

 that, as far as the electric vibrations parallel to OX are concerned, 

 the plate has exactly the emissivity that is required by Kirchhoff's 

 law. Of course, the same will be true for the vibrations in the 

 direction of OY. 



§15. It may be immediately inferred from the theorem of § 7, « 

 that the electromotive forces applied to the plate in the direction of 

 OZ, i. e. perpendicularly to the surfaces, cannot contribute anything 

 to the radiation we have considered. Indeed, we know already that 

 an electromotive force ^ex existing in the aether at the point F can 

 produce no current S~ in the plate; consequently, an electromotive 

 force ^ez iu the plate cannot cause a current Q^ at the point P. 



As to the effect produced by the electromotive force with the 



