50 GENERAL PROPERTIES OF LIVING TISSUES 



It is to be observed that each metal is placed 

 in a solution of its own salt. The ions carried to 

 the respective poles are of the same nature 

 chemically as the poles themselves, and hence do 

 not set up opposing electromotive forces when 

 they are de-ionized. 



The current produced by the Daniell cell is 

 almost perfectly constant, so long as sulphuric 

 acid still remains uncoinbined, and so long as 

 the sulphate of copper solution is kept saturated. 



Fig. 12. Rocking key, metal contact ; about one-half the actual size. 



It may be remarked that the function of the 

 porous cup is to keep the copper from depositing 

 on the zinc. 



The Pole-Changer. 1 The instrument illustrated 

 by Fig. 12 serves as a simple key, short-circuiting 

 key, and pole-changer. No mercury is used. 



1 Science, 1905, xxi, pp. 752-754. 



