Chap, xi.] NON-POLARISABLE ELECTRODES. 113 



solution, the two troughs will not be in the same con- 

 dition. If now a piece of tissue were placed upon the 

 DERIVING CUSHIONS of the non-polarisable electrodes, 

 the zinc solution would attack it, corrode it, and 

 vitiate any result. To prevent this, a piece of sculp- 

 tor's clay is made into a soft mass with a ^ to 1 per 

 cent, solution of common salt, which is a good con- 

 ductor. This is made into a thin sheet, and is folded 

 over the cushion, as shown in Fig. 58, P. It is called 

 THE CLAY GUARD. To limit the part of the clay guard 

 to be touched by the piece of tissue, a small piece of 

 thin mica may be placed on the guard. 



The clay guard is not used merely to prevent 

 corrosion and destruction of the tissue. If the animal 



Fig. 59. Non-Polarisable Tube-Electrodes. 



tissue were placed directly upon the deriving cushions 

 soaked in its zinc solution, a peculiar action would 

 take place between the liquid conductor and the 

 tissue, the result of which would be the develop- 

 ment of what is called SECONDARY RESISTANCE, which 

 would grievously diminish the intensity of any current 

 that might be present. Salt solution is found incap- 

 able of developing this secondary resistance when in 

 contact with animal tissues, while at the same time it 

 is a good conductor. 



