( 308 ) 
Physics. — “The Entropy of Radiation”. By J. D. vaN DER 
Waats Jr. (Communicated by Prof. J.D. vAN DER WAALS.) 
The entropy principle may be formulated in different ways. 
Strictly speaking, nothing follows from the examination of the 
cyclic process of CARNOT but: 
dQ. A 
Ne IF is a total differential, if the process is reversible. 
2nd, If we pass from state A to state B in a not reversible way, 
dQ. 
fF is smaller than when the process is reversible. 
The second formulation is: The entropy tends to a maximum, 
i.e. the entropy always increases; and if the other conditions to 
which the system is subjected, allow different processes, that one 
in which the increase of the entropy is greatest, will take place. 
As we may consider every slight change as “the process’, we may 
also say, that the fluction of the entropy is always a maximum. 
To derive the second formulation from the first, we must gene- 
ralize the idea entropy. We must then also attribute entropy to 
substances which are not in equilibrium. It being however imposs- 
ible, that in a reversible way a substance is brought to a state 
which is not a state of equilibrium, the definition: entropy is i 
for a reversible process, cannot be applied here. It has really been 
attempted to find a conception of entropy which also applies to 
substances which are not in a state of equilibrium. 
In order that the second formulation of the principle be correct, 
the original conception must be extended still in another way. An 
entropy of radiation must be introduced. Whether a deviation from 
the law of CARNOT might be obtained by radiation, has repeatedly 
been made the subject of an investigation. BARTOLI) imagined a 
cycle in which apparently a deviation occurred. Prof. BOLTZMANN °) 
proved however, that this contradiction may be avoided by taking 
into account the pressure exercised by luminous rays on a body by 
which they are absorbed or reflected. 
In this they tried to solve only this question: Can we obtain 
by means of radiation a process, in which the substances yield 
quantities of heat, which have another ratio than would foliow from 
1) Barron, Sopra i movimenti prodotti dalla luce e dal calore, Frrexze, 
Le Monnter 1876. 
*) Wied. Ann. XXII 1 Anno 1884. No. 5. Page 31. 
