DANIELL'S BATTERY. 37 



copper, and portions of the solid salt are placed upon the upper 

 copper plate, which is perforated like a collander for the purpose of 

 keeping the solution always in a state of saturation. The internal 

 tube is filled with the same acid mixture without the copper. A 

 tube of porous earthenware may be substituted for the membrane 

 with great convenience, but probably with some little loss of power. i 



A number of such cells may be 

 connected very readily, by attaching 

 the zinc of the one to the copper of 

 the other, and (as shown in fig. 14) 

 thus forming an intensity arrange- 

 ment of great power and constancy. 



This arrangement of battery is 

 eminently suited to all kinds of elec- 

 trical operations, and it may be 

 borne in mind that it was by operating with this battery the idea 

 of electro-metallurgy first occurred. In this battery we see that 

 the evils arising from the slow liberation of the hydrogen from the 

 surface of the negative metal, and the deposition of the zinc upon 

 the copper, and also the blackening of the surface of the copper, 

 are all surmounted. Nevertheless it is not used to any extent 

 in the art of electro-metallurgy, it being much less economical 

 than the ordinary batteries, from the quantity of copper salt 

 necessary to keep it in a working condition, and from the neces- 

 sity of using porous diaphragms, which speedily wear out. If the 

 diaphragm is made of animal membrane, the acid very soon destroys 

 it ; and although unglazed porcelain lasts a little longer, the acid 

 acts upon the alumina, so that after a few days' working the dia- 

 phragm becomes too porous ; and if the zinc plate touches the porous 

 vessel, a circumstance very difficult to avoid, there is very soon 

 formed in and upon the porous surface a deposit of copper which 

 speedily renders the cell useless, besides producing a loss of copper. 

 The saturation of the zinc solution, already spoken of, not unfre- 

 quently produces the same effects the saturated portion of the bot- 

 tom becomes reduced by the local action, and thus often a minute 

 point of metallic zinc touches the cell, and forms a nucleus for a 

 deposit of copper upon the porous cell, which spreads over the sur- 

 face very rapidly. There are always pieces of amalgamated zinc, 

 like fine scales, falling to the bottom of the cell, which also form 

 nuclei for the deposition of copper upon the porous cell. 



After the cells have been some time in use they are very liable to 

 break, if care be not taken to keep them in clean water till the salts 

 within the pores are dissolved out ; but if this precaution is taken 

 they may be preserved for a long time if only used occasionally. 

 1 Daniell's Chemical Philosphy, 2d edition, 1843, p. 504. 



