Kohlenberg—Osmosis and Osmotic Pressure. 259 
the cell and the outer liquid simultaneously can never entirely 
keep the concentration of the liquid layers in immediate con¬ 
tact with the two sides of the septum exactly the same as the 
concentration of the liquids further away from the membrane; 
yet in many cases this may be accomplished with a fair degree 
of approximation. Moreover, by using the stirring process, the 
osmotic pressures may be measured fairly approximately even 
when the amount of material passing out of the cell is not neg¬ 
ligible; in other words, when the membrane is not semiperme- 
able. 
The apparatus devised for stirring the contents of the cell 
and also the outer liquid simultaneously during the measure¬ 
ment of osmotic pressures is pictured in Fig. 7 in diagramatic 
form. In the figure, the beaker B of a capacity of 1000 cc. or 
more, contains the outer liquid. The latter is stirred by means 
of the stirrer F, which is moved up and down by the motion 
of the crank C. In the experiments performed it was admis¬ 
sible to make this stirrer of iron, for this is not attacked by pyri¬ 
dine. F consisted then of a bright, stout iron wire bent in ring 
form. Just above the cork closing the beaker, this stirrer F 
was jointed so that it would not need so large an opening in 
the cork in which to move up and down. A thermometer is 
placed in the liquid to register the temperature. The whole 
apparatus is set in a constant temperature room, or the beaker 
B is immersed in a bath of constant temperature, not shown in 
the figure. The osmotic cell S is made exactly as heretofore 
described, except that before tying on the membrane, the stirrer 
is inserted into the apparatus through the opening in the bell 
of the thistle tube. This stirrer consists of a perforated disc 
of light sheet iron fastened by riveting and a drop of solder, to 
a sufficiently stout, yet flexible, iron wire the upper end of 
which carries a lug of soft iron soldered on, as shown in the 
figure. In order that the perforated disc at the low~er end of 
this stirrer might not pound on the delicate membrane and in¬ 
jure it, prongs of wire w r ere soldered on the main vertical wire 
of the stirrer; and after the latter had been introduced into the 
osmotic cell, these prongs were bent outward in such a way that 
they would strike the side of the bell of the thistle tube (see 
