144 ELEMENTARY LESSONS ON [CHAP. in. 



silver are the two metals, the zinc being immersed in 

 chloride of zinc, and the silver embedded in a stick of 

 fused chloride of silver. As the zinc dissolves away, 

 metallic silver is deposited upon the -f- pole, just as the 

 copper is in the DanielPs cell. Mr. De la Rue has con- 

 structed an enormous battery of over 11,000 little cells. 

 The difference of potential between the first zinc and 

 last silver of this gigantic battery was over 11,000 volts, 

 yet even so no spark would jump from the + to the 

 pole until they were brought to within less than a quarter 

 of an inch of one another. With 8040 cells the length 

 of spark was only 0*08 of an inch. 



175. Marie Davy's Battery. In this cell the zinc 

 dips into sulphate of zinc, while a carbon plate dips into 

 a pasty solution of mercurous sulphate. When the cell 

 is in action mercury is deposited on the surface of the 

 carbon, so that the cell is virtually a zinc-mercury cell. 

 It was largely used for telegraphy in France before the 

 introduction of the Leclanche' cell. 



176. Gravitation Batteries. Instead of employing 

 a porous cell to keep the two liquids separate, it is pos- 

 sible, where one of the liquids is heavier than the other, 

 to arrange that the heavier liquid shall form a stratum 

 at the bottom of the cell, the lighter floating upon it. 

 Such arrangements are called gravitation batteries ; but 

 the separation is never perfect, the heavy liquid slowly 

 diffusing upwards. Daniell's cells arranged as gravi- 

 tation batteries have been contrived by Meidinger, 

 Minotto, Callaud, and Sir W. Thomson. In Siemens' 

 modification of Daniell's cell paper- pulp is used to 

 separate the two liquids. The " Sawdust Battery " of 

 Sir W. Thomson is a Daniell's battery, having the cells 

 filled with sawdust, to prevent spilling and make them 

 portable. 



177. Latimer Clark's Standard Cell. A standard 

 cell whose E.M.F. is even more constant than that of 

 the Daniell was suggested by Latimer Clark. This 



