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is considered, and not that of every one of the molecules separately, 
we may say in consequence of the entropy of radiation, introduced 
by Wren: the entropy is never lost, and it propagates continuously 
through space. In what follows I hope to prove: Entropy originates 
only, when collisions (or their analogue in radiation) occur, and the 
new entropy is then to be found at that place where the collision 
has taken place. 
Though much might be learned from considerations like those of 
Wien, I prefer to make an attempt to obtain an insight into the 
nature of the entropy of radiation by considerations analogous to 
those of BOLTZMANN for the entropy of substances. 
EE. 
When writing the preceding chapter, the treatises of Prof. PLANCK 
entitled: “Ueber irreversible Stralungsvorgänge” and his debate with 
Prof. BOLTZMANN in “Die Sitzungsberichte der Akademie der Wissen- 
schaften zu Berlin, 1897, 1898 and 1899”, were unknown to me. 
My attention was afterwards drawn to them. I found that several 
of the observations which I have made already occur in Prof. 
PLANCK’s treatises. However, as my opinion differs in many respects 
from that of Prof. Pranrcr, I think that I ought to publish the 
following paper, though I can oppose against the elaborate system 
of Prof. PLANcK only a beginning of a system according to my 
views. To make clear the course of my thoughts, I have left the 
preceding chapter unchanged. In the first place I shall have to 
vindicate, why I do not follow the method of treatment of Prof. 
PLaNncK, but follow the considerations of Prof. BOLTZMANN on the 
molecular thermal movement also for the ethereal movement of ra- 
diation. For this purpose I shall put the views of the entropy 
principle of Prof. BOLTZMANN and Prof. PLanck in sharp contrast; 
or at least what seems to me to be the view of Prof. PLANCK, for 
he does not expressively state his opinion. 
Prof. PLANCK's meaning seems to me to be the following: 
The basis of his considerations is that the entropy principle is 
correct, that is to say that the entropy can only increase. 
Now many processes which occur in nature, are not elementary 
e.g. all thermal phenomena can only be treated adequately by 
applying strict mechanic laws to the separate molecules. In order 
to find an exact law of nature, it will be necessary to consider an 
elementary process which is ruled by strict mechanic laws. Now 
the entropy can change in one direction only, the cause of which 
