( 28 ) 
deal with systems, consisting of free ether. Systems, containing electric 
masses are not absolutely excluded, but still their number is very 
small and may be neglected compared with that of the systems which 
0a 05 STO 
are devoid of these masses. As + ie En 5 is always zero, systems 
Ox 
for which this expression has another value can never occur; yet 
we may admit them in such a small number, that they have no 
influence on the results. Finally we cannot take into consideration 
systems consisting of infinite space, for a finite quantity of energy 
would spread in it and we could not have a stationary distribution 
in the ensemble. Therefore it is necessary to enclose the electro- 
magnetic energy within absolutely refleeting walls. But then it is 
necessary to add a term to 4, which expresses, that the walls reflect 
absolutely, i. e. the quantities [7] are always zero at the walls. The 
WwW 
term’ — g expresses this; 4 represents a small line in a direction 
2 . 
normal to the surface; we make this line decrease indefinitely ; do 
represents an element of area. 
If this distribution is to be for ether systems, what the canonical 
distribution is for material systems, then in the first place 1j must be 
a constant in time. For the other terms this is immediately evident, 
OTK 
so we have only to show it for the term —— 
U 
The relation, we have to prove may be written : 
dp dy 
Le pe ae Ng tog cre EE AE ee 13 
dt a dt ( ) 
We will make use of the relations 
da — es Og oh (14) 
di == Jt ded . . » A > 
zgeen EE BR Pee 
dt An \dz Oy 
and of the following relations, that may be deduced from them : 
ry af OF, OF 
— — V7 { — — ot Ste ae NN 
dt? a dy? EE cy, 
da Oene tpt 
2 Hr ee : - ee ee ers 17 
de (S+5et5) ie 
and also of the corresponding relations for the other components. 
Now we have 
( d 
ey Bes pL | FA ae 
dt dz dt\ Oz Oy 
