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must be found in an elementary process, which can take place in 
one direction only. 
In this way quite a different idea of reversibility is introduced 
as that which was originally deduced from the cycle of CARNOT. 
The reversibility according to CARNOT means, if we consider more 
closely the mechanism of the movement of heat, that all states, 
through which the system has passed, are states of equilibrium. 
These states of equilibrium now, are nothing but a particular kind 
of stationary states, namely such as can exist, without continual 
change taking place necessarily anywhere outside the system. So 
e. g. a gas between two plates, one kept at 100° by means of 
steam and the other at 0° by means of melting ice, is ina perfectly 
stationary state, which however is no state of cquilibrium, as on 
one plate steam is continually condensing and on the other ice 
melting. 
It is easy to see that this idea has little in common with the 
idea of irreversibility of Prof. PLANCK. Many processes are irreversible 
according to CARNOT, reversible according to Prof. PLANCK, e. g. 
thermal processes which are brought about by the movement of 
the molecules. In these processes Prof. PLanck grants the rever- 
sibility according to his definition. As these processes however, 
increase the entropy, it seems to me, that Prof. PLANCK ought not 
to have tried to find a process, which is irreversible according to 
his definitions but an explanation, why processes, which are irre- 
versible according to CARNOT can only cause increase of entropy. 
This observation of mine would scem fallacious only to him who 
wanted to explain all thermal processes not by molecular motion and 
collisions, but either by radiation or by an elementary strictly irre- 
versible process of which we have as yet not the least idea. Now 
we shall investigate the question whether there is really an elemen- 
tary strictly irreversible process. 
Prof. BOLTZMANN denies this positively. 
As well in the ordinary mechanics (provided heat and other 
internal movement be introduced as kinetic energy) as in all ether 
phenomena no process occurs that could not take place in an opposite 
direction. If a movement fulfils the equations of LAGRANGE and 
those of MAXWELL, the same applies to a movement which arises 
from the former by reversing all velocities and all magnetic forces. 
This observation seems to me to be quite decisive. Yet the con- 
sideration of all processes is not equally justified. The movement of 
a Herrz’s vibrator, which is damped because of the emission of 
radiation, may be thought to take place in opposite direction, so 
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