48 GENERAL PROPERTIES OF LIVING TISSUES 



As the displacement of the meniscus is propor- 

 tional to the electromotive force of the cell, it is 

 obvious that the latter has rapidly diminished. 

 The solution contains the ions of water as well 

 as those of the salt. When the circuit between 

 the platinum and zinc is completed the cations 

 H + and Na + move towards the cathode. There 

 the more easily de-ionized H + yields up its elec- 

 tricity, and hydrogen appears on the cathode. 

 The corresponding quantity of electricity is con- 

 veyed into the solution at the anode by ioniza- 

 tion of the zinc. The deposition of hydrogen 

 on the negative plate checks the electromotive 

 force setting from the zinc to the platinum in 

 two ways : first, because gas is a bad conductor, 

 and the effective surface of the platinum is 

 thereby diminished by the bubbles collecting 

 on it ; and secondly, because hydrogen is electro- 

 positive, and creates an electromotive force in 

 the direction from platinum to zinc, and thus 

 "polarizes" the cell. This new electromotive 

 force opposes the original current from zinc to 

 platinum. 



The Daniell Cell. Daniell discovered an elec- 

 tro-chemical method of avoiding polarization, and 

 thus was able to construct a cell that would 

 furnish a current of unvarying strength. In the 

 Daniell cell the two metals employed are zinc 



