VOLTAIC GENERATORS. 27 



and the former arrangement is the most advantageous in 

 this respect. 



In Bunsen's cell nitric acid acts as a depolarizing agent, 

 that is to say, as a means of preventing a deposit on the 

 carbon of the hydrogen resulting from the decomposition of 

 the water by the zinc. When this deposit is formed on the 

 carbon, there is, in fact, an electro-motive force developed 

 in consequence of the reaction of the hydrogen on the liquid, 

 which force, acting in the contrary direction to that of the 

 battery, weakens the latter the more the longer the current 

 is in action, and the less resistant is the circuit. Now nitric 

 acid, being very rich in oxygen, and readily giving it up, 

 permits the hydrogen disengaged in the cell to combine with 

 that oxygen, and, so to speak, intercepts it on its passage 

 before it is able to produce its bad effect. On the other 

 hand, this acid, by itself acting chemically upon the acidulated 

 water, develops an electro-motive force which acts in the 

 same direction as that produced by the oxidation of the zinc, 

 and this still more increases the energy of the battery. 



The inconveniences of this battery are its expense and the 

 suffocating and unhealthy emanations from it. The nitric 

 acid, on being deoxidized, passes into the state of nitrous 

 acid, which is liberated in the form of reddish-brown vapours 

 that are difficult to prevent. 



In order to avoid too great a consumption of zinc, and 

 to render the action of the battery more constant, the metal 

 is amalgamated, the special purpose of this operation being 

 to annul the local couples which are due to the impurity of 

 the zinc, and consume it without any useful result. The 

 simplest means of obtaining this amalgamation is to plunge 

 the zinc into a liquid composed in this way : 



Two hundred grammes of mercury are dissolved by heat in 

 1,000 grammes of aqua regia (nitric acid i, and hydrochloric 

 acid 3). When the solution of the mercury is complete, 

 1,000 grammes of hydrochloric acid are added. 



The zinc is allowed to remain for a few seconds in this 



