xii CONSTRUCTION OF SIMPLE VOLTAIC CELLS 193 



agent. Consequently, when the free hydrogen comes into contact 

 with the nitric acid the chemical change which occurs is that the 

 nitric acid gives up some of its oxygen, and this combines with 

 the free hydrogen to. form water ; or, as it is commonly expressed, 

 the hydrogen is oxidised to water by the oxygen of the nitric 

 acid, which is itself reduced, first to nitrous acid and eventually 

 to the lower oxides of nitrogen. Thus is the free hydrogen got 

 rid of. 



Leclanche"s Cell. This cell (Fig. 100), which we shall describe 

 because of its familiar use in houses for working electric bells, is 

 different in two or three respects from 

 those which have already been dealt with. 

 There is no acid used in it. The liquid 

 which acts upon the zinc plate Z is a 

 solution in water of a salt called ammonium 

 chloride or sal-ammoniac. The result of 

 the chemical changes which take place 

 when the liquid acts upon the zinc is that 

 a compound known as a double chloride 

 of zinc and ammonium is formed, and two 

 gases, ammonia and hydrogen, are evolved. 

 To get rid of the hydrogen the positive 

 pole C of the cell, which is made of 

 carbon, is surrounded with black oxide 

 of manganese, a compound rich in oxygen. 



The carbon plate and the fragments surrounding it are enclosed 

 in a porous pot P, like that of the Daniell's cell. The hy- 

 drogen which it is necessary to get rid of combines with some of 

 the oxygen of the manganese compound and is changed into 

 water, while the manganese compound is gradually reduced. 



Positive and Negative Poles. In each one of the cells 

 which has been described the chemical action in the cell has started 

 at the zinc plate, and has proceeded from molecule to molecule 

 throughout the liquid until the copper plate, or its equivalent, 

 has been reached. The plate where the chemical action begins, 

 and the electric current starts in the liquid, is called the 

 negative (-) pole. The plate where the chemical action in 

 the cell is completed and where the current starts outside the 

 liquid is called the positive ( + ) pole. These terms are easily 

 understood by reference to Fig. 94, which represents a closed 

 circuit, as it is called. If the wires from the two plates are 



o 



