CH. I.] DIALYSIS. 3 



below. Pour this back into the stock, revolving the tube with the 

 mouth downwards so that an even film is left adherent to the walls 

 of the tube. Add another portion of collodion solution and repeat 

 as before. Allow the film to dry, so that it does not stick to the 

 finger. When this point is reached fill the tube with cold water. 

 Cut round the rim of the tube with a knife, pour off the water, and 

 carefully detach the membrane from the side of the tube. Allow 

 water to run between the sac and the glass. By means of a glass 

 rod with a spatulate end, and by traction and twisting, the sac can 

 usually be removed from the glass tube. Fill the sac with water. It 

 should be perfectly transparent. A cork, bored with a large hole, 

 can be tied into the upper end, and by means of this it can be sus- 

 pended in a jar of distilled water. The secret of success is to fill the 

 tube with water at a particular moment, determined by trial on each 

 specimen of collodion. If the water be added too soon the sac is 

 opaque and feeble. If it be added too late, it is somewhat difficult 

 to remove the film from the glass without damage. Sacs prepared 

 in this way are very much better than those made of parchment 

 paper. They should be kept wet, as on drying they become porous. 



NOTE. Preparation of Collodion Solution. To 3 gm. of gun cotton 

 (pyroxylin) add 75 cc. of ether and allow to stand for 10 or 15 minutes in a flask 

 closed with a cork. 25 cc. of ethyl alcohol are then added, and the pyroxylin 

 dissolves to a mobile fluid, which does not require nitration. It should be 

 allowed to stand until all bubbles have disappeared. 



2. Dialysis. Mix 2 per cent, starch paste (see Ex. 135) with 

 about one-tenth of its volume of saturated ammonium sulphate 

 solution. Place the mixture in a collodion sac and suspend this in 

 water contained in a tall jar. Care must be taken to avoid spilling 

 any of the mixture into the jar. Examine the " dialysate " (the 

 fluid in the jar) for starch by the iodine test (Ex. 136) and for am- 

 monium sulphate by means of barium chloride at the end of half an 

 hour. The starch test will probably be negative, whilst the sulphate 

 test will be positive. The dialysate should also be examined for 

 starch after 2 to 7 days. 



C. Osmotic Pressure. 



Certain membranes can be prepared which allow of 

 the passage of water molecules, but do not allow dissolved 

 substances to pass through them. Such membranes are 



