Medway — Material and Shape of the Rotating Cathode. 181 



Since the crucible should in any event be weighed before 

 each determination, such small loss does not seriously affect 

 the availability of the silver crucible as a substitute for 

 platinum. 



Similar experiments were made with a nickel crucible of 

 50 cm3 capacity, under a procedure exactly the same as that 

 described, with the results as recorded. 



Deposition upon Nickel. 



Copper Copper 



taken. found. Error. Current. N. D. Time, 



grin. grm. grm. Amp. -100. Min. 



(1) 0-1041 0*1028 —0-0013 1-5 5* 15 



(2) 0-1041 0-1054 +0-0013 2« 6'6 12 



(3) 0*1041 0-1036 —0-0005 2' Q-6 15 



These results show that while nickel may be employed as a 

 cathode, too much reliance must not be placed upon results 

 obtained by its use when the greatest exactness is required. 

 Care must also be taken in drying the crucible, since nickel is 

 very easily oxidized with a consequent increase in weight. The 

 deposit of copper was removed from the. crucible with nitric 

 acid, with considerable loss of nickel, as the following will 

 show : 



I. II. III. 



Weight of nickel crucible before 



treatment.. 17*6478 17-6161 17'6091 



Weight of nickel crucible after 



treatment ___ 17-6161 17'6091 17*5932 



Loss of nickel 0*0317 00070 0*0059 



So it appears that, while the silver crucible may with some 

 economy and without sacrifice of accuracy be substituted for 

 the platinum crucible used as a rotating cathode in the elec- 

 trolytic determination of copper, the ease with which the cru- 

 cible of nickel is attacked, both during the analytical process 

 and in the subsequent removal of the deposit, is a bar to the 

 use of that metal for the rotating cathode. 



Shepherd* recommends the use of the ordinary disc anode 

 of platinum as the rotating cathode, in place of the platinum 

 crucible, while a strff platinum wire, carried in semi-circular 

 conformity to the edge of the disc cathode, serves as the anode. 



According to my experience with this form and adjustment 

 of apparatus, the deposits obtained are not so adherent as 

 might be desired and tend to crumble away from the edge of 

 the disc ; and the same thing is true of discs of copper and 

 silver. The probable reason for failure to obtain an adherent 



* Jour. Phys. Chem., vii, 508, 1903. 



Am. Jour. Scl — Fourth Series, Yol. XYIII, No. 105. — September, 1904. 

 13 



