( 742 ) 
as they are often declared to be. And if the current opinion — 
which certainly greatly contributes to the greater popularity of the 
osmotic pressure compared with that of the thermodynamic potential 
— were really correct, that we can form a clear idea of the 
physical meaning of the first quantity and ot of the second, then 
there could not be any doubt for him which method to prefer, if 
for the rest the circumstances were quite the same. 
But this. current opinion seems to me hardly tenable and on the 
other hand I believe that in many respect the thermodynamical 
potential is preferable to the osmotic pressure, and that therefore it 
will be advisable to put the question whether it would not be 
better to return to the older method both for scientific investigation 
and for instruction ? 
§ 2. This question has lately again been put forward by Mr. 
VAN LAAR in an address for the “Bataafsch Genootschap” at Rotter- 
dam *), which was followed by an article “Over tastbare en ontast- 
bare grootheden” (On palpable and impalpable quantities) *). Though 
1 readily admit, that these papers have induced me to consider the 
problem of the osmotic pressure specially, there would not be any 
reason for me to diseuss Mr. van Laar’s views here, when only 
his address had appeared. For IT can fully subseribe to the general 
tendency of this paper though of course TL would not be responsible 
for every statement, as moreover has already appeared from my 
preceding communications in these Proceedings — and L should 
therefore only have to consider what in my opinion would have to 
be added to his address. His second paper, however — and in 
this I have specially in view $ § 6 and 7, pointed out as the gist 
of his paper by the author himself — Mr. van LAAR seems to me 
to harm rather than to promote the good cause, which he has 
espoused with so much ardour, and already for this reason 1 feel 
it incumbent upon me to protest against this part of his reasoning. 
I think that 1 accurately represent the gist of it as follows: It is 
true that we cannot form a clear idea*) of the nature of the ther- 
modynamie potential, but we cannot do so for the osmotic pressure 
either. Nor is this surprising, for the improved philosophical insight 
of the last years gives us the conviction that our natural philosophy 
never works with any but fictitious (though sharply defined) ideas, 
1) Also published in Chem. Weekblad, 1905, N° 1. 
2) Chem. Weekblad, 1905, NO, 9. 
3) Mr. van Laar speaks of a “palpable conception” (tastbaar begrip). It would 
lead me too far if I would account for the reason why I think that IT may, 
nay even ought to substitute the term chosen here for it. 
