136 



ELEMENTARY LESSONS ON [CHAP. in. 



they can only be made use of in zinc-carbon or zinc- 

 platinum cells. Nitric acid also attacks zinc when the 

 circuit is open. Hence it cannot be employed in the 

 same single cell with the zinc plate. In the Bichro- 

 mate Battery, invented by Poggendorf, bichromate 



of potash is added 

 to the sulphuric acid. 

 This cell is most con- 

 veniently made up as 

 a " bottle battery " 

 (Fig. 72), in which a 

 plate of zinc is the 

 pole, and a pair of 

 carbon plates, one on 

 each side of the zinc, 

 are joined together at 

 the top as a + pole. 

 As this solution acts 

 on the metal zinc 

 - when the circuit is 

 open, the zinc plate 

 is fixed to a rod by 

 which it can be drawn 

 up out of the solution 

 when the cell is not being worked. Other cases of 

 chemical prevention of polarisation are mentioned in 

 describing other forms of battery. 



3. Electro-chemical Means. It is possible by employ- 

 ing double cells, as explained in the next Lesson, to so 

 arrange matters that some solid metal, such as copper, 

 shall be liberated instead of hydrogen bubbles, at the 

 point where the current leaves the liquid. This electro- 

 chemical exchange entirely obviates polarisation. 



166. Simple Laws of Chemical Action in the 

 Cell. We will conclude this section by enumerating the 

 two simple laws of chemical action in the cell. 



I. The amount of chemical action in the cell is propor- 



Fig. 72. 



