In Investigations on the theory of Brownian movement, he says that osmosis holds for solute particles but not for Brownian ones, since they are too big to pass through a permeable membrane.
So what? Why should this prevent such Brownian particles to exercise pressure?
Later on, one reads
But a different conception is reached from the standpoint of the molecular-kinetic theory of heat. According to this theory a dissolved molecule is differentiated from a suspended body solely by its dimensions
In my view, this argument seems totally uncorrelated from the previous one, even though in the paper they are contiguous. I don't understand the reasoning.