232 



APPENDIX 



be subdivided and dipped in sections. Let the tracing be lowered 

 quickly into the solution and after a few seconds taken out and 

 drained. If it be now laid upon a sheet of filter paper (or a news- 

 paper) it will be dry in a few minutes. 



9. NON-POLARIZABLE ELECTRODES. 



The Du Bois-Reymond non-polarizable (N. P.) electrode is made as 

 follows (Fig. 86): t, glass tube of about 4 mm. lumen; z, zinc rod 

 with binding screw (the zinc rod must be amalgamated before use 

 in an electrode) ; r, rubber tube clasping both glass tube and zinc 

 rod; zs, saturated solution of sulphate of zinc, introduced with a 

 narrow-pointed pipette; K, kaolin plug, made by working China- 

 clay powder into a stiff paste with normal salt solution. 



FIG. 86 



Electrodes: A, kaolin electrode ; 2, zinc rod ; 2.<?, saturated solution of ZnSO*; t, glass tube ; 

 r, rubber tube ; K, plug of plastic kaolin ; B, v. Fleischl's brush electrode, in which a camel's- 

 hair brush is substituted for the kaolin plug ; C, hand electrodes, made by pushing the com- 

 mon battery wires through rubber tubing for insulationand binding together with thread. 



The electrodes should be filled at each time of using, and the 

 parts may be " assembled" in the order and manner enumerated 

 in the description. 



The Fleischl brush electrode differs from the foregoing in substi- 

 tuting the brush of a camers-hair pencil for the kaolin plug. This 

 variation of the non-polarizable electrode is somewhat more difficult 

 to prepare, but is more convenient for certain uses. 



Porter's Non-polarizable Electrode. This electrode is boot-shaped 

 and is the most convenient form for general laboratory use. The 

 electrode is of porcelain with glazed leg and unglazed foot. 



To prepare the electrode soak it in normal saline solution for an 

 hour or two to fill the pores of the unglazed portion with the salt 

 solution. Fill the foot of the boot with a saturated solution of zinc 

 sulphate. The amalgamated zinc rods may be dropped into the 

 legs of the boots and the electrodes are ready for use. 



After the laboratory period pour out the ZnSO 4 and put the boots 

 in running water or in a considerable excess of water for twelve to 



