558 ILL USTRA TI VE EXPERIMENTS 



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 in. 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 tlie 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 ')iot 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 treat- 

 ment 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, 

 is filled with collodion sol, care being taken to pour the fluid slowli/ down the 

 side so as not to form air-bubbles. The sol is now poured back into the stock. 

 This should be done slowly and steadily with a constant rotatory motion 

 of the flask, leaving a thin film adherent to the glass. If this operation is 

 carried out too quickly, the layer of collodion at the bottom of the flask 

 will be too thin. It is convenient to allow a little collodion to overflow 

 all round the outside of the neck of the flask to enable one to get a good 

 grip in pulling out the film afterwards. The flask (acetate sols are at this 

 stage submersed in water) is inverted over the mouth of a bottle containing 

 a little alcohol (or empty, see above), and allowed to dry as before. When 

 dry, the flask is filled with water, or, better still, immersed in water and 

 allowed to stand for at least 15 minutes. The collodion sac is loosened 

 at the neck and carefully withdrawn. This is a slow process and must 

 not be hurried or grave risk will be run of tearing the thin film. It is quite 

 a good thing to remove the film in stages, letting it soak in water between 

 times. The complete bag is usually fitted with a glass mouth — a piece of 

 glass tubing of suitable size inserted in the neck and affixed thereto by broad 

 rubber bands, or by wrapping in oiled silk, gutta-percha tissue, etc., and 

 winding thin string over this, or by causing it to adhere with a little fresh 

 collodion. 



(iii.) To make collodion test tubes, {a) Collodion tubes may be formed 



