SCIENCE AND PRACTICE. 223 



and on the sides of the porous pots. Sometimes, after a 

 battery has stood unused for a while, the porous pots are 

 found to be completely impregnated with metallic copper, 

 filling up their pores, and forming short circuits between 

 the solutions, reducing thus the action of the elements to 

 almost nothing, while the consumption of zinc is, at the same 

 time, increased. 



This deposit of metallic copper is not the result of galvanic 

 action, but of cementation. It would not occur if the plates 

 were made of pure metal. As the common zinc is dissolved 

 in forming the currents, the particles of iron and other metals 

 mixed with it, fall to the bottom, and separate the copper 

 from the solution of its salt as the latter comes through the 

 pores of the diaphragm. A small local element is thus 

 formed which goes on reducing the metallic copper and 

 adding to the bulk of the deposit. 



The method suggested by M. Place which is now employed 

 to lessen this damaging effect, is to saturate the bottoms of 

 the porous pots to the height of a quarter of an inch with hot 

 wax or parafine, and, in setting up the battery, to fill the dilute 

 sulphuric acid into the elements four or five hours before 

 putting in the sulphate of .copper solution. The pores of 

 the pots become well filled with acidulated water before the 

 sulphate comes into contact with them. No precaution can, 

 however, entirely prevent this detrimental property. 



Another cause of inconstancy in the action of Daniell's 

 element arises from the solution of copper entering the 

 chamber appointed for the solution of zinc. When this 

 occurs the trespassing copper is precipitated out of the 

 sulphate, and adheres to the zinc cylinder, the colour of 

 which changes to red and black, and its electro- positive 

 condition becomes weakened. The electro-motive force of 

 the element is lessened in proportion as the copper covers 

 the surface of the zinc ; and the quantity of sulphate of 

 copper and metallic zinc consumed represent a much greater 

 strength of current or length of time that the circuit has 

 been closed, than the operator has really had the benefit of. 



Both these processes of destruction of the element take 



