PREPARATION OF GELS 563 



(ii.) For Mett'fi tubes. Suck up some of sol 89 (d) or undiluted blood seruiu 

 stained with methylene blue or gentian violet into capillary tubes (about 

 2 mm. bore). See that the aspirated sol is free from air bubbles. Lay the 

 tubes horizontally on a piece of gauze, wrap them and tie the gauze into a 

 bag. Immerse the tubes in water (whicli has been boiled and allowed to 

 cool to 85° C.) for 2-3 minutes. Cool and dry the tul)es on filter paper. Dip 

 the extreme ends in melted paraffin to seal the tubes. When required cut 

 the tubes into lengths of about 1 cm., discarding about 1 cm. from each end. 



92. Concentrated Gelatin. 



Sheets of Coignets "Gelatin A" washed for 48 hours in three changes of 

 ice-cold water acidified to ^H 4-6 originally (falling to 4-9 eventually). 

 Warm (below 60° C), and it dissolves in its own water of imbibition. Keep 

 liquid under 40 c.c. Add some toluene as preservative. 



93. Non-polarisable Electrodes. 



Brush electrodes of FleiscM (for Experiments 54, 55, etc.). The electrodes 

 consist of glass tubes (4 cm. x 5 mm.). In one end fits a perfectly clean 

 camel's-hair pencil, and into the other dips a well-amalgamated rod of zinc 

 with a binding screw at its free end. A piece of indiarubber tubing fits as a 

 cap over the upper end of the tube, holding the zinc rod in position. 



Preparation of the electrode. Make a paste of kaolin, using normal saline 

 solution as the moistening agent. Pack the paste into the tube to cover the 

 fixed end of the brush and to give a layer of about ^ cm. in thickness. 



(At this stage in the preparation the electrodes are placed in normal saline 

 solution until required for use.) 



When required the tubes are filled with a saturated solution of ZnS04 and 

 the zinc rods are inserted. 



■M—-J. 



