134 ELEMENTARY LESSONS ON [CHAP. in. 



pasty amalgam. The iron particles do not dissolve in 

 the mercury, but float up to the surface, whence the hydro- 

 gen bubbles which may form speedily carry them off. As 

 the zinc in this pasty amalgam dissolves into the acid the 

 film of mercury unites with fresh portions of zinc, and so 

 presents always a clean bright surface to the liquid. 

 If the zinc plates of a battery are well amalgamated 

 there should be no evolution of hydrogen bubbles when 

 the circuit is open. Nevertheless there is still always a 

 little wasteful local action accompanying the action of 

 the battery. Jacobi found that while one part of 

 hydrogen was evolved at the positive pole, 33*6 parts of 

 zinc were actually dissolved at the negative pole, instead 

 of the 32-5 parts which are the chemical equivalent of 

 the hydrogen. 



163. Polarisation. The bubbles of hydrogen gas 

 liberated at the surface of the copper plate stick to 

 it in great numbers, and form a film over its surface ; 

 hence the effective amount of surface of the copper plate 

 is very seriously reduced in a short time. When a 

 simple cell, or battery of such cells, is set to produce a 

 current, it is found that the strength of the current after 

 a few minutes, or even seconds, falls off very greatly, 

 and may even be almost stopped. This immediate 

 falling off in the strength of the current, which can be 

 observed with any galvanometer and a pair of zinc and 

 copper plates dipping into acid, is almost entirely due to 



' the film of hydrogen bubbles sticking to the copper pole. 

 A battery which is in this condition is said to be 

 " polarised." 



164. Effects of polarisation. The film of hydro- 

 gen bubbles affects the strength of the current of the cell 

 in two ways. 



Firstly, It weakens the current by the increased resist- 

 ance which it offers to the flow, for bubbles of gas are 

 bad conductors ; and, 



Secondly, It weakens the current by setting up an 



