CONSTRUCTION OF A MEIDINGER CELL. 689 



'time the deep blue colour changes into a lighter blue, a few more 

 crystals must be added. 



A battery of Meidinger elements when first put together should 

 be left to stand for a few hours with closed circuit, before making 

 use of it, for its action is extremely feeble at first. 



When the battery after long use ceases to act, it should be taken 

 to pieces. The zinc is then generally found to be so thin and full 

 of holes that it crumbles during the attempt to clean it. But if, 

 after scraping off the impurities which are usually found deposited 

 upon it, a pretty firm cylinder of zinc is left, it ought to be well 

 cleaned with a hard brush, and used once more after amalgamating 

 it again if necessary. At the bottom of the vessel a quantity of 

 copper is found, partly as a brown soft sediment, partly as a 

 dense bright red deposit upon the copper plate, the latter of which 

 may be removed by bending the copper plate to and fro. The greater 

 part of the deposit will peel off, and after scraping off the remainder 

 as completely as possible, the copper is bent into its original form. 

 The soft sediment is of course poured into a vessel if, as is the 

 case in large electric telegraph establishments, it is intended to sell 

 the copper residues. They repay in such cases the outlay for the 

 cupric sulphate, but where the batteries are used on a small scale 

 as in a laboratory, the sale of the copper residues is scarcely feasible. 

 In taking the cells to pieces the two liquids cannot be prevented 

 from mixing, hence the cells must be refilled with a fresh solution 

 of Epsom salt. 



The copper cylinder is sometimes, especially in Meidiuger ele- 

 ments sold by dealers, replaced by a cylinder of lead. The 

 action of lead in a galvanic cell is less energetic than that of copper, 

 and hence such cells produce at first a very feeble current ; but as 

 copper is very soon deposited on the lead, the action is afterwards 

 the same as if the whole cylinder were made of copper, while on the 

 other hand the leaden cylinders are more easily bent than those of 

 copper, and the removal of the deposited copper is therefore easier. 



The precipitation of metals from solutions by the 

 action of the galvanic current has received many im- 

 portant practical applications. If the precipitation pro- 

 ceeds sufficiently slowly a dense coherent layer of metal 

 is formed, which adapts itself exactly to the external 

 surface of the cathode, and under certain circumstances 

 adheres to it very firmly. Thus if a conductor is irn- 



y Y 



