CHEMICAL ACTION IN THE GALVANIC CELL. 683 



the action of the current escapes in the form of bubbles, 

 but another part is gradually condensed upon the 

 copper, platinum, or carbon respectively, as an invisible 

 fine layer, which covers the entire surface of the plate. 

 It follows that the points of contact of plate and liquid 

 are more and more reduced, and that hence the quantity 

 of electricity produced becomes gradually less. More- 

 over, the film of hydrogen thus formed acts precisely like 

 a metal with reference to the liquid which is interposed 

 between it and the zinc, an electromotive force begins 

 to act, which is not only perfectly independent of that 

 previously called into play by the two metals and the 

 liquid, but which tends to generate a current opposite 

 in direction to that produced by the battery. The result' 

 is that the current is rapidly enfeebled and that batteries 

 arranged like those represented in figures 336 and 346 

 only act energetically during the first few moments after 

 the circuit is closed. 



The deposition of hydrogen on the surface of the 

 plate which forms the positive pole must consequently be 

 prevented by some means if a constant current is to be 

 obtained. Elements in which this is eifected are called 

 ' constant ' elements. They generally contain two liquids, 

 or in a few rare cases they contain besides the two 

 plates which form the poles, one liquid and one solid, 

 the latter of which covers the positive plate and is 

 acted on chemically while the current exists. Bunsen's 

 and Grove's elements are constant ; in either of them 

 the hydrogen which is produced decomposes the nitric 

 acid in the porous cell, and the products of the decom- 

 position are water and nitrous acid, the latter of which 

 does not interfere with the action of the elements. 



