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3ACKLUND '), who seems?) to require even ether waves to explain it, 
chiefly two theories have been developed about the nature of the 
osmotic pressure: the static and the kinetic theory. The first theory 
finds warm advocates in Pupin *) and BARMWATER *); it seems however 
doubtful to me whether they have closely realised the consequences of 
their assertions. At least the latter brings forward as an objection 
to the kinetic nature of the osmotie pressure: “Ein molekulares 
Bombardement in einer Fliissigkeit ist mir immer etwas sonderbar 
vorgekommen’’; notwithstanding he considers the equation of state 
of Van per Waars by no means as a “sonderbar” instance of false 
ingenuity, but as an example to be followed. However this may be, 
he who does not want to break with all our conceptions about 
heterogeneous equilibrium, will not be able to explain such an equili- 
brium in another way than statistically i.e. as a stationary condition 
of a great number of moving particles. This does, of course, not 
detract from the fact that the question may be put what forces are 
required to bring about that state of equilibrium. This implies that 
the adherents of the static theory need not be altogether mistaken 
when they assert that the cause of the osmotic pressure is to be found 
in forces of attraction. On this point I shall add a few remarks at 
the end of this communication. 
§ 2. Of much more importance than this static theory of the osmotic 
pressure is the kinetic theory. The great majority of its advocates 
(I shall speak presently about the few exceptions) take as their basis 
the equality, which bas been proved experimentally and by means 
of thermodynamics, of the osmotic pressure and the gas pressure (the 
pressure which the molecules of the dissolved substance in the same 
space would exercise, when they were there alone and in rarefied 
gas state) and derives from this that they have both the same cause 
in this sense that the dissolved substance is present in the two cases 
in the same state and so acts in the same way; this is then 
expressed in about this way that the solvent converts the dissolved 
substance into the rarefied gas state. This conception seems doubly 
remarkable to me; first because it seems to be pretty well generally 
prevailing *), secondly because it alone seems to me to be able to 
1) Lunds Univ. Aarsskrit 40. 
4) I know his paper only from an abstract in the Beibl. 29, 375. 
3) Diss. Berlijn 1889. 
4) Diss. Kopenhagen 1898 and Zsch. phys. Ch. 28, 115. 
8) It is naturally difficult to give a proof of this opinion, therefore I shall only 
adduce the following citations as a confirmation. 
“If we look a little more closely into the matter, we find that in the case of 
dilute solutions, at least, there is far more likelihood of the dissolved substance 
