and the Electro- Chemical Equivalent of Copper. 87 



From these experiments we may conclude that a moderate 

 change of temperature is not a critical matter. 



The First and Second Series were made at the temperature 

 of the room which did not vary much from 19° C. 



The following method is recommended for the preparation 

 of the solution : Dissolve copper nitrate, the purest obtainable, 

 in distilled water. Boil for a short time to expel the air and 

 keep at a temperature of 100° C. for an hour or two in the pres- 

 ence of bright metallic copper in order to completely neutralize 

 the solution. When ready for use it should have a density of 

 about 1-53. Add a saturated solution of NH 4 C1 at about the 

 rate of 1 drop to 100 cc of the nitrate. Just the proper amount 

 of ammonium chloride it is not easy to specify. If too much 

 be present the deposit will discolor on drying ; if too little, 

 bubbles will collect on the cathode or the deposit will oxidize 

 in the solution. Perhaps the surest way is to try a prelimi- 

 nary experiment, adding the NH 4 C1 till the plate begins to 

 show discoloration on drying. 



If the solution is to stand the anode may be left in it. Care 

 should be taken to exclude the air. 



On removal from the solution the cathode should be washed 

 thoroughly in running water and drained until it may no longer 

 be said to be wet, after which gentle heat should be cautiously 

 applied until all moisture is removed. 



The advantages possessed by the nitrate may be enumerated 

 as follows : 



1st. The weight of copper deposited does not appear to 

 depend on the current density. 



2d. It permits the use of a high current-density. 



The limit is not easily stated. When 25 amperes per 

 square centimeter is reached the cathode needs watching, for it 

 will fringe at the edge but does not become powdery. 



No way was found to prevent the branching with a high 

 current-density. 



A greater amount of NH 4 C1 should be present when the 

 current density is increased. 



3d. The weight of copper is practically independent of the 

 temperature between 10° and 35° C. 



The tendency to oxidation is increased by lowering the tem- 

 perature but this may be easily counteracted by the addition 

 of more ]S T H 4 C1. 



4th. The solution may be used a number of times. 



5th. The equivalent agrees to four figures with that calcu- 

 lated from the best chemical determinations, viz : 



Richards Eq. =(3294'4±0'1) X lO^ 



First Series Eq. = (3294'S±0'43) X 10-» 

 Second Series Eq. =(3293-9 ±0*39) X lO^ 1 



