MEMBRANES 407 



is filled with collodion sol, care being taken to pour the fluid slowly 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 is loosened at the neck and carefully withdrawn.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 

 inside test tubes or boiling tubes in the same manner as has been described 

 for sacs. 



(6) Sometimes it is desirable to make a dialysing tube which will fit on 

 the outside of a specified tube. A test tube of the required width is taken 

 and a small hole blown in the end. This hole is covered with a thin film 

 of collodion which is allowed to dry. The whole tube is then dipped in the 

 collodion solution. The excess collodion is allowed to drip into stock, the 

 test tube being held bottom upwards at an angle of less than 45 and 

 rotated steadily. The film is treated with water, or after drying as above, 

 depending on the nature of the solution. After soaking in water, the test 

 tube is filled with water and the film gently worked off like a tight glove. 

 It is eased a little at the bottom, water meanwhile passing into the collodion 

 tube through the hole in the test tube. Little by little a fine film of moisture 

 will creep up between film and glass and the transparent collodion tube 

 will slip off easily. This method yields tubes which are more uniform in 

 thickness and permeability than the former. 



(iv) Thimbles. Strong dialysing vessels may be made by impregnating 

 Soxhlet extraction thimbles with collodion. The thimble, soaked in 

 alcohol, is immersed in alcohol-ether collodion, withdrawn, allowed to drip 

 and partially dried, and then the process of impregnation is repeated. It 

 is advisable to fix a short glass tube into the mouth of the thimble before 

 impregnation and to use a fairly thin collodion solution. 



II. Parchment Paper is sold for this purpose in sheets, or made up in 

 long tubes (sausage skins) or in thimbles. When dry the paper becomes 

 very brittle, so that great care has to be observed in its storage to prevent 

 creases, etc. 



(i) Sheet. Select a piece of the paper free from obvious defects (pin- 

 holes, etc.) and fully two inches larger in each direction than is required. 

 Soak in water till soft and pliable. Place it centrally over a dialysing glass 

 as is described above for collodion sheets. The folding of the free edges 



