2 EXPERIMENTAL PHYSIOLOGY 



Grenet's Battery (fig. 2) is a single fluid battery. It consists of 

 an amalgamated zinc plate fixed between two carbon plates k, k. 

 The zinc plate is fixed above to a rod b, by means of which it can be 

 lifted from the fluid. The two carbon plates are connected to the 

 binding screw, e, which is therefore the positive pole ; the zinc is. 



Fig. 1. — The Dakiell Battery. 



Fig. 2. — The Gkenet Battery. 



connected to d. The fluid is made by adding four parts of a 10 per 

 cent, solution of potassium bichromate to one of sulphuric acid. In 

 action the zinc is dissolved, and the hydrogen set free at the carbon 

 plates is oxidised by the bichromate and thus removed. "When 

 freshly made the battery has an E.M.F. slightly above 2 volts, but 

 rapidly falls until it reaches about 1*8 volts. 



A Bunsen Battery (fig. 3) consists of an outer earthenware pot in 

 which is placed a zinc cylinder. Inside this is a porous pot carrying 

 a square block of carbon, c. The wire connections are made to the 

 carbon, the positive pole, and to the zinc, the negative pole. The 

 porous pot is filled with strong nitric acid, and the fluid surrounding 

 the amalgamated zinc is dilute sulphuric acid (1 to 7). The S0 4 

 appearing at the zinc plate when the battery is in action dissolves 

 the zinc to form ZnS0 4 , and the H._> appearing simultaneously at the 



