ii4 PHYSIOLOGICAL PHYSICS. [Chap. xi. 



A second form of non-polarisable electrodes is seen 

 in Fig. 59. A flattened tube of glass r contains a 

 slip of amalgamated zinc. The end of the tube is 

 closed by the moistened sculptor's clay, and the tube 

 itself is filled with the zinc solution. The clay pro- 

 jects from the end of the tube, and the projecting part t 

 can be made of any shape, and can be sharply pointed 

 so as to touch just a point of the tissue to be examined. 

 The tube is mounted on a universal joint h, and sup- 

 ported on a brass upright s. These electrodes are not 

 only suitable for leading off 'a current to the galvan- 

 ometer, but also for leading currents to nerve or 

 muscle. They are free from polarisation, even after 

 being used for hours. A nerve to which they have 

 conducted a current is not, therefore, injured by 

 products of electrolytic action, which would have col- 

 lected at the poles of ordinary metallic electrodes. 



To amalgamate the troughs or slips of zinc 

 the best fluid is Berjot's amalgamating fluid. The 

 directions for making it are as follows : " Dissolve 

 at a gentle heat 200 grammes of metallic mercury in 

 1,000 grammes of a mixture of one part, by weight, of 

 nitric acid, and three parts of hydrochloric acid, and 

 then add 1,000 grammes of the last-mentioned acid." 

 When not in use, the liquid should be kept in a well- 

 stoppered bottle, and placed in a cool dark place, to 

 prevent decomposition. 



The resistance offered by such non-polarisable 

 electrodes is considerable. To reduce it to the mini- 

 mum, the cushions of both troughs should be soaked 

 for several hours in the zinc solution, then gently 

 squeezed to express excess of fluid and air bubbles. 

 They should fit the troughs as accurately as possible, 

 so that the layer of zinc solution required to fill up 

 the trough is not great. Further, the clay guards 

 should be so placed on the cushions that no bubbles of 

 air may be between them, for this would greatly add 



