( 420 ) 



if Ave write w}' for the development of heat in the element S, which, 



h 



in the state A, is dne to the current in the principal direction h. 



Now, starting from the expression (50), we shall obtain the total 

 value of {^ipY by an addition, in which all elements s, each with 

 its three principal directions, must be taken into account. In a 

 system, completely shut off from surrounding bodies, ^ icj^ will be 



the total amount of energy, emitted by F in the state A ; we can 

 therefore determine it by the formula (48), putting « S =: 1. This 

 leads to the result 



IQ 71 k c^ n dn 



In the same way, using the theorem of § 7, /; and the expression 

 (49), I find 



1 6 rr /t c^ n^ dn 



(»,/.)' = ;-3 • 



O q V 



These results being independent of the place of the point P and 

 the choice of the direction /, we come to the conclusion that the 

 state of things is the same in all parts of the medium L and that 

 both the electric and the magnetic vibrations take place with equal 

 intensities in all directions. The amount of the electric and magnetic 

 energy per unit of volume is now easily found. According to § 4 

 the first is 



for the value of which one linds 



A 7t kc^ dn 



by remembering that for every direction /, 



Tiie magnetic energy may likewise be determined. Referred to unit 

 volume it has the value 



Y?[(^:B.)^ + cv + (^^-rj' 



and this is easily calculated, since for every direction /, 



{^,y = -i- (^/)^ 



n 

 The result is that the two kinds of energy are distributed over 

 the body / with equal densities. This has been known for a long 



