( 309 ) 
the law of Carnor? Wien has first introduced an “entropy of 
radiation” '), He thinks it a matter of course, that radiation which 
can be in equilibrium with radiating bodies, and which possesses 
energy, must also possess entropy. He derives his arguments exclusively 
from the examination of reversible processes. He defines as “tem- 
perature of radiation” the temperature of a perfectly black body, 
which is in equilibrium with this radiation. In reversible processes, 
however, the quantity of heat yielded by the walls is the same as 
that communicated to the ether. As further according to the defini- 
tion the temperatures of the walls and of the radiation are the 
same, it comes to the same thing whether the law of Carnot be 
applied to the ether, as Wien did, or to the walls, als BOLTZMANN 
did: { 7 38 identical in both cases. The necessity of the con- 
ception “entropy of radiation” can therefore never be concluded from 
reversible processes. 
E. Wriepemann had already pointed out the necessity of that 
conception for phosphorescence- and fluorescence phenomena °). 
Yet it is clear that if the entropy principle is expressed in the 
second formulation, every irreversible radiation phenomenon is in 
contradiction with the entropy principle, if we do not attribute en- 
tropy to radiation. Every body which radiates heat into a vacuum, 
which heat is not at the same time absorbed by another body, 
would lose entropy without that at the same time at least an equal 
amount of entropy was gained elsewhere. Therefore the entropy 
principle requires, that the ether participating in the movement of 
radiation, is assumed to have at least as great an amount of entropy, 
as the radiating body has lost. Whether it is possible to find such 
an entropy function for radiation, cannot in my opinion, be doubted. 
This extension of the entropy principle is l:ss hazardous than that 
in which the second formulation is derived from the first. Yet 
nobody will doubt whether the second formulation is correct, pro- 
vided that we follow BOLTZMANN in considering the entropy prin- 
ciple not as an exact law but as a principle of probability. 
Wren derives, inter alia, from his considerations, the theoretical 
reliability of the law of STEPHAN and the relative intensity of the 
different wave-lengths in the light emitted by black bodies. 
Another advantage of his introduction of the idea of “entropy of 
1) Wied. Ann. 52,1. Anno 1894, No, 5. P. 132 sequ. 
2) Wied. Ann. 38,3, Anno 1889. No. 11, P. 485, 
