FLUXES OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
455 
Now here 
qV.U = 4E.qV.D + iD.qV.E, 
so that the terms in the third pair of brackets in (144) represent 
U, + qV.U, = ^ = iE| c E, 
dt dt 
with the generalised meaning before explained. So finally 
p\ 
EJ 0 + HG 0 = Q+ U+ T + div q(U + T) + ^ (U e + T M ) 
+ (U div q — E.DV.q) + (T div q — H.BV.q), . . . (145) 
which brings (142) to 
e,,Jo -f-h u G 0 = Q + U + T + div {W + q (U + T)} 
+ g- (U c + Tn) + (U div q — E.DV.q) + (T div q — H.BV.q), . . 
. (146) 
which has to be interpreted in accordance with the principle of continuity of energy. 
Use the form (127), first, however, eliminating Fq by means of 
div 2 QLq = Fq + 2 QVq, 
which brings (127) to 
e 0 J 0 + h 0 G 0 = G + U + T -j- div {W + q (U + T) q — 2 QiVq + Sa ;.(147) 
and now, by comparison of (147) with (146) we see that 
riTT 
- Sa + iQVq = (E.DV.q - U div q) - 
+ (H.BV.q - T div q) - ~ 5.(148) 
from which, when p and c do not change intrinsically, we conclude that 
G n = B.HN - NT + D.EN - NU, 
P N = H.BN - NT + E.DN - NU, 
(149) 
as before. In this method we lose sight altogether of the translational force which 
formed so prominent an object in the former method as a guide. 
Some Remarks on Hertz’s investigation relating to the Stresses. 
§ 22. Variations of c and p in the same portion of matter may occur in different 
ways, and altogether independently of the strain variations. Equation (146) shows 
