1 8 PHYSIOLOGICAL PHYSICS. [Chap. n. 



The gas settles in minute bubbles upon the surface of 

 the copper plate, and at once interferes with the 

 action in the battery. It interferes both by the resis- 

 tance it offers to the passage of the current and also 

 by setting up a current in an opposite direction, 

 which tends to weaken the original current by 

 neutralisation. This action is called polarisation of 

 the plates. Besides this, in such an element some of 

 the sulphate of zinc produced in the element is 

 attacked by the hydrogen, and deposited on the copper 

 plate. So that the copper plate begins to approach 

 the condition of the zinc plate, and, of course, the 

 difference of potential becomes reduced. In all these 

 ways the current is diminished. Thus such an ele- 

 ment is not of constant strength, but rapidly gets 

 weakened. To meet these difficulties various elements 

 have been devised, which have been called, therefore, 

 constant elements. Some of these will be immediately 

 described. 



Amalgamated zinc. In most elements zinc is 

 one of the metals employed. Chemically pure zinc 

 is, however, very dear, and impure zinc is unequally 

 attacked by acid. The impure zinc gets quickly 

 eaten through with holes, and, by this, local currents 

 are produced in the zinc plate itself, which interfere 

 with the main current. To rectify this, ordinary 

 rolled zinc is employed, but before use it is amalga- 

 mated in the following way : It is first washed with the 

 dilute sulphuric acid (1 to 7) to get a bright surface, 

 and then rubbed all over with mercury. The mercury 

 forms a bright coating all over the surface. Thus 

 coated, the zinc is not attacked by the dilute acid, 

 unless connection is made between it and the other 

 metal of the element, or, in other words, unless the 

 circuit is closed. Local currents are thus prevented. 

 Where zinc is mentioned as part of an element, it is 

 understood to be amalgamated. 



