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certain limited time excluded from external influence, and may be 
considered to be in equilibrium. During the time, however, that the 
equilibrium exists, every particle must on an average absorb as much 
energy as it emits. So an inward radiant vector must exist during 
that time. 
But, says Ritz, when we consider a finite system, we must always 
think it enclosed within walls which contain a finite number of electrons, 
and which therefore can reflect the radiation in fewer ways than 
Lorentz’s totally reflecting walls can. So Lorenrz’s theory is not 
applicable to the natural systems. This really offers a difficulty. 
Even the y-rays of radium pass through metal sereens in a consi- 
derable degree, and it is possible that vibrations of still smaller 
wave-length possess so great a penetrating power that they are never 
in equilibrium of radiation in our experiments. Yet it seems to me 
that there are facts which indicate that in Rrrz's observation, which 
is quite correct in itself, the clue to the explanation of the normal 
spectrum is not to be found. For it seems to me that if Rivz’s 
explanation was the true one, the spectral formula of RAYLEIGH 
would have to hold for all wave-lengths which are still regularly 
reflected by the walls, which is by no means the case. Moreover, 
we should then have to expect that this formula would be fulfilled 
with the greater degree of approximation as the walls were thicker, 
and so more wave-lengths were approximately in equilibrium of 
radiation; then we should not find a definite spectral formula in- 
dependent of the thickness of the walls. We may finally imagine 
the walls to be infinitely thick, so that they would contain, an 
infinite number of electrons, and Ritz has not shown that also in 
this case his restriction of the number of possibilities in the inward 
radiation originating from the walls, is justified. 
While Rrrz’s theory deals only with the normal spectrum, JEANS 
tries at the same time to find a solution of the difficulties attached 
to it, and of the difficulties attached to the specific heats. He, too, 
thinks that he has to find the solution in the fact that supposition 
B is not satisfied. He thinks, namely, that the coordinates of a 
system may be divided into two kinds: 1 those which we may call 
conservative coordinates, which possess an appreciable kinetic energy ; 
2 those which we may call dissipative coordinates, which can receive 
energy from the conservative ones only exceedingly slowly, and 
which lose the energy they have received so rapidly by radiation, 
that they never possess an appreciable quantity of energy. Then the 
kinetic energy which must be ascribed to systems agrees with their 
number of conservative coordinates and so is less than would cor- 
