408 ILLUSTRATIVE EXPERIMENTS 



requires some care to ensure even lying against the side of the glass. Tie 

 with thin string. Test for leaks. 



The sheet may be made into a bag as follows : " Cut a regular hexagon 

 and soak it thoroughly in water. Then place it centrally on the bottom 

 of an inverted beaker or jar, the diameter of which is about one-third of 

 that of the inscribed circle of the hexagon. Gently pinch radial folds 

 from the circumference of the beaker to the corners of the hexagon and 

 mould them so that the paper midway between the corners touches the 

 wall of the beaker, and then turn the folded portions over and smooth them 

 into cylindrical shape. The folds must not be sharp, as even wet parch- 

 ment may be damaged by too drastic treatment. When the bag has been 

 moulded as described, a string is loosely tied around it, or a fairly slack 

 rubber band slipped over it within about 2 inches of the edge, and the bag 

 is then drawn off the beaker. Its permanent shape is secured by threading 

 a clean thin string through the folds, which is gently drawn tight after 

 every completed stitch so that the circumference at the open end is roughly 

 the same as at the bottom. The bag is suspended in a jar of suitable size 

 by two or three strings tied at equal distances to the string which secures the 

 circumference." (From Hatschek's Laboratory Manual, Messrs. Churchill.) 



(ii) Similar care must be taken when working with parchment tubes or 

 thimbles. The " sausage skin dialysers " are excellent for demonstration 

 purposes, as they offer a large effective surface. They are sold flab in lengths 

 of about a metre and are very easily damaged. They are best kept hanging 

 from one end. Thoroughly soak and test a piece. Cut it to a convenient 

 length and with a large cork- borer excise a circular piece from both ends 

 about \ inch from the opening. Bend the tube into U -shape and place 

 it in a tall cylinder. A glass rod longer than the diameter of the cylinder 

 thrust through the holes at the ends of the dialyser acts as support. The 

 tube may now be filled, by means of a funnel, with the fluid to be dialysed 

 while the cylinder is filled with water at the same time and at the same rate. 

 This is to prevent undue strain on the tube. 



20. Dialysis. In using any dialyser it is advisable to look to the following 

 points. 



1. Test for leaks. 



2. See that neither the preservative nor the fluid to be dialysed act on 

 the substance of the membrane, e.g. bile pigment increases the permea- 

 bility of collodion. (Bile may be dialysed through a double dialyser, i.e. a 

 collodion tube suspended in a larger tube of the same material. Blood 

 pigment occasionally presents the same difficulty.) 



3. If the dialysate is wanted as well as the dialysed fluid, dialysis must 

 be carried out by changing the external fluid from time to time. 



4. If it is not necessary to keep the external fluid for examination, rapid 

 dialysis may be obtained by keeping up a continuous flow of water in the 

 outer vessel. This is most conveniently done by placing the dialyser or a 

 series of dialysers in a sink, the level of water in which may be adjusted 

 by means of a wide glass tube running through the waste plug. The water 

 supply is led to the bottom of the sink. 



(1) Dialyse (a) Egg albumin + Sodium Chloride. (6) Starch + Iodine + 

 HC1. (c) Starch + Glucose. Test dialysate for both constituents. 



(2) Using a glass dialyser with a collodion membrane dialyse a mixture 

 of either congo red, litmus or ali/arin and hydrochloric acid. 



