( 746 ) 
meant by Van per Waars implies equality of that used by us in 
the definition, and that we may therefore substitute the latter for 
the former in the definition of the thermodynamic potential. 
§ 5. In this way we have obtained a clear idea of the nature 
of the thermodynamic potential, which so far as I can see is in no 
respect second to that of temperature, external pressure, kinetic 
pressure, number of collisions, mean length of path ete. That for 
all this it is not always easy to derive in a special case the value 
of the thermodynamic potential from this kinetic meaning is self- 
evident, as well as that it will possibly always be more desirable 
to derive the thermodynamic potential by means of thermodynamic 
functions than from direct kinetic considerations. It is true that we 
do not avoid the latter in this way either, but we make use of the 
result of these considerations, as it is given in the equation of state. 
In these two respects, however, the thermodynamic potential is in 
no way inferior to the osmotic pressure, as appears from my two 
preceding Communications, specially from $ 8 of the second. 
§ 6. Mr. van Laar informs us, that in connection with his address 
he had been asked “to supply something as a substitute” for the 
osmotic pressure and the kinetic conception of it, something that 
“conveys some meaning”’.') This request seems by no means so 
unreasonable to me as it seemed to Mr. van Laar and I think that 
I have complied with it in the preceding pages. Now I may be 
allowed to prove that this “something else” is at the same time 
“something better”. 
First of all it seems not very appropriate to me to give a quantity 
of pressure such a prominent place in the theory of mixtures. As 
soon as we deal with this theory in general, i. e., include also 
external forces and capillary phenomena (which are very often of 
great importance, I need only mention critical points) it appears, 
that the pressure is a quantity we may only handle with great 
caution and which may certainly not be treated as fundamental 
variable.*) In a much higher degree this objection holds for the 
osmotic pressure. For this is, as we have seen, nota quantity which 
is characteristic for the state in which the solution is; the peculiar 
laws of the osmotic pressure are not due to the fact that matter 
in dilute solution is in a particular, peculiar condition, they originate 
— in their generality — only from our arbitrariness, which by means 
of fictitious ideas, calls peculiar conditions into existence on paper, 
which never exist in reality. For let us not close our eyes to this 
1) Chem. Weekblad 1905 No. 9, § 3. The inverted commas are Mr. van Laar’s. 
2) Cf. the footnote on p. 735. 
