434 
Brown. — On the Preparation and 
a test-tube. A full account of the method of preparation has been given in 
the paper already cited, and it is unnecessary to repeat the details here. 
Though the writer prefers to make the thimbles on the outside of the test- 
tube, as greater uniformity in successive membranes can be obtained by that 
method, it is probable that the alternative method will be found preferable 
for general purposes, as a certain amount of practice is required for success 
in stripping the membranes from the outside of the tube. A brief account 
was given in the original paper of a method of preparing membranes inside 
the test-tube, and it was stated then that these membranes tended to be too 
thin and fragile for practical purposes. Subsequent work has, however, 
shown that that difficulty can be overcome. 
Schering’s celloidin was formerly used in this work ; latterly a prepara- 
tion known as ‘ Necoloidine 5 , and supplied by the New Explosives Company, 
has been found to be equally satisfactory. For preparing membranes by 
the ‘inside 5 method, the arrangements given on p. 615 of the paper already 
mentioned will be found to give good results with the following alterations. 
Use a stronger solution of collodion (12 per cent.), and reduce the time of 
drainage to about two to three minutes. The membranes tend to be some- 
what thin at the upper (closed) end, but this tendency can be checked by 
careful warming of the upper end of the tube during drainage. This has 
the effect of vaporizing some of the ether from the upper part of the 
membrane and preventing further running of the collodion sol in that part. 
After drainage the membrane is dried off completely by blowing air 
into the tube. It is then separated from the tube by filling the latter with 
water when it is found that the collodion membrane readily separates from 
the glass. The membrane is now dried off completely, when it is ready for 
the next stage. 
The important points to observe in drying the membrane inside the 
tube are : (1) not to touch the membrane until drying is well advanced, 
otherwise a hole will result ; (2) to continue blowing in air until the membrane 
is well dried and may in fact begin to separate from the tube of its own 
accord. If the water is added before drying is well advanced, the subsequent 
drying in the open air will entail considerable shrinkage and distortion. 
2. The membrane in its air-dried form is highly impermeable, and 
therefore, from the point of view of an osmometer, insensitive. To increase 
its permeability without at the same time making it permeable to the more 
slowly diffusible crystalloids, it is placed for twenty-four hours* in (about) 
60 per cent, alcohol — the maximum percentage varies somewhat with 
different makes of celloidin — then washed in several changes of water until 
the alcohol is removed. It must be emphasized that from the time the 
membrane is taken out of the grading mixture of alcohol and water it 
must not be allowed to undergo any drying. If dried out, it will of course 
revert to its previous highly impermeable condition. 
