Kohlenberg—Osmosis and Osmotic Pressure. 245 
goes through the septum is practically a negligible quantity. 
From what has been said above, it appears that the latter quan¬ 
tity is never absolutely nil, and that consequently there is really 
no such a thing as a semi-permeable membrane, strictly speaking. 
Now as has been intimated, the qualitative experiments above 
detailed enable one to foretell when a membrane will permit 
the solvent to go through so much more readily than the solute 
that the amount of the latter which traverses the membrane 
is so slight that the septum may be called semi-permeable. 
From the ether, water,, chloroform experiment, already de¬ 
scribed as a typical case of osmosis, it appears that the ether 
makes its way through the water into the chloroform because 
(1) ether is soluble in water and (2) chloroform has much 
more affinity for ether than has water, so that ether is extracted 
from the water layer by the chloroform; on the other hand, the 
water does not permit chloroform to pass into it and into the 
ether beyond to an appreciable extent, because chloroform is so 
very slightly soluble in water even when the latter is impreg¬ 
nated with ether. 
Holding these things in mind let us look for the proper solv¬ 
ent and solute to employ with rubber as the membrane so that 
the latter shall be seminermeable. It must first be remembered 
that rubber (vulcanized caoutchouc) is practically a hydro¬ 
carbon. The rubber employed was of excellent quality and was 
almost the pure gum. On analysis it was found to contain 0.38 
per cent ash, and a Carius determination yielded 0.30 per cent 
chlorine and 0.95 per cent sulphur. Before the analysis was 
made the sample was wiped superficially, washed with distilled 
water, dried with filter paper and finally left in a desiccator 
over strong sulphuric acid for twenty-four hours. It is evi¬ 
dent that in order to pass through a rubber membrane a sub¬ 
stance must be taken up by the rubber, the rubber must imbibe 
the substance, in other words the substance must be soluble in 
rubber. 1 Again the liquid bathing the other side of the rubber 
must be capable of extracting the imbibed substance from the 
iThe act of such solution or imbibition is really mutual; i. e. the 
rubber attracts the substance in question, and the latter in turn at¬ 
tracts the rubber. 
