406 ILLUSTRATIVE EXPERIMENTS 



for general use. They are, as ordinarily prepared, unsatisfactory for the 

 dialysis of whole blood or of bile. 



Cellulose nitrate (gun-cotton or pyroxylin) is generally sold damped with 

 alcohol and, for very accurate work, should be dried before weighing. For 

 the following experiments this refinement is unnecessary. 



Acetic Acid Collodion. Four grams of commercial gun-cotton are placed 

 in a wide-mouthed glass-stoppered bottle of 200 c.c. capacity ; 100 c.c. 

 glacial acetic acid are added. The mixture is shaken gently until the gun- 

 cotton has dissolved, leaving no residue. The resulting sol is transparent 

 and will keep for weeks. 



Alcohol-Ether Collodion. (Bigelow and Gemberling.) 75 c.c. of ethyl 

 ether are poured on 3 gms. of gun-cotton in a wide-mouthed stoppered 

 bottle as above. After 10 to 15 minutes, 25 c.c. of ethyl alcohol are added 

 and the mixture agitated until a clear solution is obtained. 



Either of these collodion solutions (after standing till free from bubbles) 

 may be used in the preparation of membranes. 



(i) To cover a vessel similar to a Graham dialyser. Cleanse a piece of 

 plate glass thoroughly and polish it. Pour sufficient collodion sol on the 

 centre of the plate to give a large enough membrane. Avoid bubbles. By 

 tilting the plate, spread the sol evenly over the surface of the plate and, 

 if necessary, drain off any excess into the stock bottle, taking care to avoid 

 any unevenness in the distribution of the collodion. The diameter of the 

 sheet should be fully 2 inches larger than that of the dialysing glass. If an 

 acetate sol has been used, immediately plunge the glass and adherent gel 

 into cold water and leave it immersed for about half-an-hour to convert 

 the acetate gel into a hydrogel. After the minimum time of immersion 

 has elapsed, the collodion film may be readily detached from the glass, 

 placed centrally over the rim of a dialysing cup, and fixed in place by a 

 broad rubber band. The dialyser should now be tested for leaks by filling 

 it with water and observing that no fluid escapes round the junction. The 

 film must be kept moist or it will shrink and rupture. 



The ether-alcohol gel is not placed in water at once like the above, but 

 is allowed to dry in air or in alcohol vapour for a period depending on the 

 permeability required. If a very permeable diaphragm is required, a glass 

 trough is inverted over the film so as to prolong the period of gelation. 

 Although the degree of drying is the crucial point of the whole process, no 

 definite rules can be laid down. Each " make " of collodion requires 

 treatment on its own merits and experience alone will tell when the film 

 is ready. If the sol has been made as directed, it should be dry enough 

 when it does not stick to the finger when touched lightly. The edges can 

 be loosened with a spatula or paper knife and the whole film slowly lifted 

 from the glass. When about three-fourths of the sheet has been raised 

 vertically from the plate the rim of the dialysing cylinder is placed below 

 it so that the edge of the rim comes in contact with the collodion surface 

 which has been next the glass plate, i.e. with the surface which has been 

 dried least. The edges of the membrane are carefully turned down over 

 the sides of the cup and will adhere quite firmly. If desired, a broad 

 rubber band may be placed round the rim to ensure tightness. Test, as 

 above, for leaks and leave immersed in water for 10-15 minutes to allow 

 the alcohol to be replaced by water. 



(ii) To make a collodion sac. A small Erlenmeyer flask, clean and dry, 



