( 729 ) 
we neglect in our formula the terms, which are multiplied by a by 
the side of those in which this is not the case, if we put po=Peoer. 
and if we take woor. instead of 7, which is permissible for very 
dilute solutions we get: 
MET 
P = po—Peoex, = — —— log| 1—«a 
Veoer. 
which gives the well-known formula of vaN r Horr when the og 
is developed and the higher powers omitted. 
I wish to point out, that also a more accurate treatment yields 
the logarithmic form which Borpinen and van Laar have advocated 
— and there could not be any doubt but it must be so — but that 
it also shows that van Laar’s statement’) was too absolute when 
he asserted that a correction term need never be applied in the 
numerator voe (Or Vo) in Connection with the size of the molecules. 
In the second place I draw attention to the fact that we find the 
osmotic pressure exclusively expressed in what VAN DER Waars has 
called thermic quantities (in opposition to caloric quantities). It 
appears to be unnecessary to take into consideration the heat of 
dilution or other quantities of heat, which van ’r Horr *) seems to 
deem necessary for concentrated solutions and which Ewan *) has 
taken into consideration. Even if we had avoided all the introduced 
neglections, so when we had not assumed, that the vapour follows 
the gaslaws, nor that v,=v,, may be put in some terms, nor that 
the area C’ may be neglected compared to A, nor (the most important) 
that 4 is constant, we should evidently not have had to deal with 
any quantity of heat. This seems important to me, as both theore- 
tically and experimentally the caloric quantities are much less accessible 
than the thermic ones. 
Physics. — ‘“‘Ainetic derivation of vax ’r Horr’s law for the 
osmotic pressure in a dilute solution.” By Dr. Pu. Konnsraum. 
(Communicated by Prof. vax per Waats). 
§ 1. When we leave out of account the more intricate theories 
as that of PorrriNG ©), who tries to explain the osmotic pressure 
from an association of solvent and dissolved substance, and that of 
My Ibe: 
2) K. Svenska Vet. Ak. Hand. 24. Quoted by Ewan Zsch. phys. Ch. 14 
409 en 410. 
8) Zsch. phys. Ch. 14, 409 en 31, 22. 
4) Phil. Mag. 42, 289. 
