PARCHMENT DIALYSERS 559 



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 lit 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 degrees 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 is immersed in alcohol-ether collodion, withdrawn, allowed to drip and 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. 



79. Parchment Dialysers. 



Parchment 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 2 in. 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 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 parchment 

 Biay 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 in. 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 ilat in lengths 

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



