270 Wisconsin Academy of Sciences, Arts a/nd Letters. 
solute will pass through to a notable extent; if on the other 
hand, the solute has very little affinity for the septum, as is 
the case with cane sugar, the membrane will allow but very 
small quantities to pass through, and hence will be “ semi- 
permeable. ” It is perfectly clear, too, that, in general, dif¬ 
ferent precipitated membranes would lose part of their 
w r ater with different degrees of readiness, and hence in quan¬ 
titative measurements of osmotic pressures different results 
would be obtained when different precipitated membranes are 
used. This is shown to be true by the experiments of Pfeffer 
(Osmotische Untersuchungen) who found that when copper 
ferrocyanide membrane was used a one per cent sugar solu¬ 
tion yielded an osmotic pressure of 51.0 cm. at 14.2°; when a 
Prussian blue septum was employed the pressure was 38.7 cm. 
at 13.9° ; and when a calcium phosphate membrane wa.s used 
the pressure was 36.1 cm. at 15.2°. In all three cases the 
sugar which passed through the septa was insignificant ac¬ 
cording to his determinations. While the results of Pfeffer 
are not final as determinations of the maximum osmotic pres¬ 
sures, inasmuch as he did not stir the liquids while measur¬ 
ing the pressures, the experiments nevertheless serve to show 
that the pressures developed with the different septa are by no 
means the same. If one were to compute the molecular 
weight of sugar from the osmotic pressures which a one per 
cent aqueous solution of it develops when copper ferrocyan¬ 
ide, Prussian blue and calcium phosphate are used as mem¬ 
branes respectively, different values would obviously be ob¬ 
tained, that is the conclusion would be reached that the mo¬ 
lecular weight of sugar in one and the same solution varies in 
different cases, which is obviously absurd. The experimental 
facts are, of course, readily explained by what has been said 
above. 
Further, when precipitated membranes are used and the 
osmotic process goes on very rapidly, it generally occurs that 
the solution robs the precipitated hydrate of water faster than 
the latter is taken up from the pure water bathing the other 
side of the septum. In such cases the membrane develops 
ruptures through which the solution oozes out of the cell, 
hence the necessity of the usual practice of adding the “mem- 
