undeniable fact — and least of all should they do so who are so 
averse to “hypotheses” — that though all those semipermeable walls 
may be realised in a few cases, vet we have on the other hand thou- 
sands and thousands of eases, where we have not the slightest 
foundation for belief in their existence. What reason can there be 
for assuming, that there will ever be found a wall permeable to 
toluol, but not to benzol, and another wall, permeable to benzol 
and not to toluol, and what else is it but a fiction to speak of a 
wall, permeable to cane-sugar and not to water. (For also this is 
necessary, see Van ’r Horr, Vorlesungen II, 24). And let us even 
put the most favourable case: that such walls existed really, does 
it not remain perfect fiction then to try and treat the theory of 
concentrated solutions with them? We need only bear in mind that 
steel, our strongest material, however thick it is taken, can hardly 
bear pressures above 5000 atms, what to think then of a semiper- 
meable wall for which such a pressure is but a trifle. And now I 
do not in the least object to such fictitious ideas when they are quite 
unavoidable — this is sufficiently proved by what precedes — but 
what is the use of using them, when we have another quantity of 
simple signification, which 7s characteristic of the condition in which 
the mixture is, which can be defined solely from the properties of 
the substance with which we have to deal? 
To this comes another difficulty. He who works with the osmotie 
pressure — history teaches it — is but too apt to consider a mixture 
not as an individual, which must be examined in itself and must be 
known from itself, but as another substance (solvent), more or less 
modified by the presence of the ‘dissolved substance”. In this way 
we lose quite sight of the fact, that the two components in a mixture 
are present in exactly the same condition (the singular theory of the 
“oaslike nature” of the dissolved substance proves it); we begin to 
overlook, that ‘dissolved substance” and “solvent” are perfectly arbitrary 
names, which have only a right to existence when we confine 
ourselves to one of the two extreme cases; we are led to try and 
explain the properties of a substance from those of another, which 
is often in quite different circumstances; we begin to apply all 
kinds of hazardous approximations and compromises; we get to the 
most extraordinary association and dissociation theories. How fruitful 
on the contrary the opposite method is, the whole work of Van Dur 
Waats, the experimental and theoretic material (inter alia on the 
behaviour of mixtures with respect to the law of corresponding 
states) gathered specially at Leiden may prove. 
§ 7. Now one may object to this, that all these are theoretical 
DL 
Proceedings Royal Acad. Amsterdam. Vol. VII. 
