b 



132 Dr. J. A. Fleming on the Use of DanielVs Cell 



more serious deviations, and though more convenient is not 

 quite so reliable as electroplating the rod with fresh copper, 

 but is preferable to employing simply a scraped or glass- 

 papered surface of rolled copper. A large series of determi- 

 nations were next made on the effect of oxidation of the copper 

 surface. If a copper rod, newly electroplated, is left in the 

 copper-sulphate solution, it gradually gets oxidized and 

 covered with brown patches of oxide, and if amalgamated, 

 the mercury sinks in and the surface gets brown and patchy. 

 When this is the case the electromotive force rises, and by 

 uncertain amounts, depending on the degree of oxidation. It 

 is very remarkable how small a trace of brownish oxidation on 

 the surface raises the electromotive force several parts in 

 1000, amounting in some cases to nearly 1 per cent. .The 

 following is one set out of many experiments on this point: — 

 Two solutions of zinc and copper sulphate were prepared of 

 the pure crystals. The specific gravity of the zinc sulphate 

 was 1'4, and that of the copper sulphate was 1*1. A rod of 

 pure unamalgamated zinc and one of electrotyped copper was 

 also taken. Keeping all other conditions constant, the rod of 

 copper was first cleanly and carefully electrotyped over with 

 a new fresh surface of copper, and exhibited no trace of oxi- 

 dation; and the B.M.F. of this cell was then taken against a 

 carefully tested standard Clark cell of known value ; and the 

 rod was then suffered to oxidize on the surface by successive 

 exposure to the air, the E.M.F. of the cell being taken at 

 each stage. 



E.M.F. of cell. 

 Copper, perfectly pure, unoxidized . . 1*072 volt. 



„ slightly oxidized, brown spots. 1*076 „ 



,, more oxidized 1*082 „ 



„ covered with dark-brown oxide 



film 1-089 „ 



„ cleaned, replated with fresh 



pinkish electro-surface . . 1*072 „ 



Many other experiments of this sort showed the same thing. 

 A Raoult cell was formed by taking two beakers, one con- 

 taining zinc sulphate of specific gravity 1*2, and the other 

 copper sulphate of the same specific gravity, and using amal- 

 gamated cast zinc and electro-deposited copper rods. The 

 beakers were connected, when required, by a piece of clean 

 cotton-wick dipped in water, and connecting both solutions by 

 dipping into each. The copper pole was first freshly electro- 

 plated, and the following values obtained for the E.M.F. of 

 the cell as it gradually oxidized : — 



