Use of Collodion Osmometers . 435 
In setting up this membrane as an osmometer use is made of the fact 
that whereas collodion will not adhere to glass in presence of water, it will 
under these circumstances adhere strongly to rubber. For the present 
purpose, rubber tubing is preferable to the ordinary rubber cork as the 
cylindrical shape of the former permits of a better joint. The method of 
procedure is given below. 
A piece of pressure rubber tubing, about 1 inch long, is inserted into 
the open end of the thimble so that the edge of the membrane is about half- 
way up the ‘ cork \ The rubber tubing is chosen of such a diameter that it 
can just be inserted. A piece of ordinary glass tubing (about % to 3 inches 
long) is now passed through the cork until its inner end is flush with the 
inner surface of the latter. If the various parts have been suitably selected, 
the cork will be closely pressed against the membrane, all irregularities of 
which are thereby evened out so that a tight junction is effected. The 
membrane, filled with water to just below the lower end of the cork, is 
now set upright in water, so that the water-levels inside and outside are 
approximately the same. In this way the part of the membrane in contact 
with the rubber cork is allowed, to dry out completely by exposure to the 
air, after which the junction is painted over with a collodion solution. This 
is in turn allowed to dry off and the junction is now watertight. A short 
length of capillary tubing is attached, and the osmometer is complete. 
The simplest method of filling the osmometer is by pinching the mem- 
brane to drive out the air and running in the required solution from 
a funnel. The operation of filling must be carried out rapidly to avoid any 
risk of drying the membrane. For accurate work several fillings and 
emptyings are required each time a new solution is tested in the osmometer. 
As the membrane substance is tough it will stand repeated fillings and 
emptyings without damage, granted moderate care is exercised. Before using 
an osmometer for the first time it is advisable to keep it for a few days 
under an internal pressure of about two feet of water in order to even out 
any wrinkles that may be present, and to make sure that the membrane is 
properly sealed to the cork. 
3. These osmometers are used in class work in the following manner. 
They are filled with the solution the osmotic pressure of which is to be 
determined (either unknown concentrations of cane sugar or copper sulphate, 
or a plant extract such as beet juice), the level of the meniscus being 
arranged somewhere in the middle of the capillary tube. The osmometer 
is now placed in a succession of dilutions of a standard cane-sugar solution, 
and by a process of trial and error one determines at what degree of dilution 
of the standard solution the meniscus remains steady. For class purposes 
the normality of the unknown solution is determined to within one unit in 
the second decimal place but a greater degree of accuracy than this is 
feasible. The method of procedure described below is more systematic, and 
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