DANIELL'S ELEMENT. 685 



TZ, fixed by binding screws to the copper and to the 

 zinc, serve for connecting the elements in series. 



In this cell the hydrogen which is disengaged simply 

 replaces the copper which is deposited upon the positive 

 plate in consequence of the decomposition of the copper 

 sulphate, the result of this substitution being hydrogen 

 sulphate, that is, sulphuric acid. This sulphuric acid 

 tends to replace that used up by the zinc, while the 

 crystals of cupric sulphate on the shelf keep the solu- 

 tion of cupric sulphate, which obviously would other- 

 wise become weaker by the decomposition that is going 

 on, in a saturated state. 



The use of the porous vessel is attended in all batteries with some 

 inconvenience, and especially so in Daniell's battery, in consequence 

 of an incrustation of copper which is gradually formed upon the 

 clay, and renders the vessel useless. Batteries have therefore been 

 devised in which the porous vessel is entirely dispensed with, and 

 the separation of the liquids effected by their difference of density. 

 Such batteries are called gravity batteries. A single cell of this 

 kind, being a modification of Daniell's combination by Meidinger, 

 and known by the name of Meidinger's balloon element, is repre- 

 sented in section in fig. 348 A. 



The glass vessel g g, of which the upper portion is wider than the 

 lower, contains a thin cylinder of copper, &, and also one of zinc, z z, 

 which either rests on the shoulder formed by the junction of the 

 narrower part with the wider, as in the figure, or is suspended from 

 the edge of the glass vessel by three small projections which are 

 bent outwards and fit into three corresponding incisions of the edge 

 of the glass. The zinc is immersed in this cell in a solution of 

 magnesic sulphate (Epsom salt) in water, and the copper cylinder 

 in cupric sulphate ; this being heavier than the magnesic sulphate 

 fills the lower part of the vessel, and is therefore in contact with 

 the copper cylinder, while the magnesic sulphate, which is lighter, 

 remains in the upper part of the vessel round the zinc cylinder. 

 The shelf in Daniell's cell is replaced by the balloon Z>, which con- 

 tains a store of crystals of cupric sulphate, by which the solution, 

 which without it would gradually become weakened, is constantly 

 kept saturated. In charging the cell the glass g g is filled to about 



