Kohlenberg—Osmosis and Osmotic Pressure. 255 
that the difficulty lay in the membrane itself, the initial experi¬ 
ments having been performed by using the sheet rubber with¬ 
out further treatment; but actual tests showed that whether 
the rubber was previously extracted with boiling hot pyridine 
or not made no perceptible difference in the results. It will be 
noted that one set of experiments was made using a common 
thick sheet rubber (No. 56). In this set the results were very 
different from those obtained with the thin rubber of high 
grade (No. 53), and yet the duplicates did not differ from each 
other more than when thin rubber was used. It was also de¬ 
termined by several trials that the non-concordance of the 
results of duplicate experiments could not be laid to the fact 
that in some cases the rubber was stretched rather more than 
in others. It was not to be expected, of course, that the max¬ 
imum pressure would be reached in the same time in two ex¬ 
periments that were similar, for the areas of the surfaces of 
the membranes and their thickness were not exactly the same. 
It might further be possible that the differences in pressure 
observed in the experiments that were duplicates of each other 
occurred because slightly different amounts bf sugar passed 
through the different membranes; in other words, that there 
was more leakage of solute in one case than in the other. Such 
leakage or lack of semi-permeability would operate to diminish 
the osmotic pressure in two ways, (1) by directly letting ma¬ 
terial out of the cell as the pressure rises, and (2) by increas¬ 
ing the amount of solute in the outer liquid and so weakening 
the cause which creates the pressure. It is quite true that in 
all cases small quantities of sugar passed through the mem¬ 
branes, and that these were, perhaps, not always exactly the 
same. The qualitative tests made, however, always showed that 
the quantity wteich had passed through was very small and far 
below the amount required to produce a noticeable osmotic pres¬ 
sure with the apparatus employed during the time of the ex¬ 
periments. It will be recalled that 1.2 per cent solution of 
sugar in pyridine yields practically no osmotic pressure (No. 
25) and that the same is true of a 0.05 normal solution of silver 
nitrate in pyridine at room temperature (No. 20). Further¬ 
more, when two similarly charged osmometers were set in one 
