CHEMISTRY: MacINNES AND CONTIERI 
323 
irregularities in the surface of the iron the points are scattered, but the 
trend is clearly the same as that of the overvoltage-pressure curves 
given in the previous paper. This appears to be a promising method 
of increasing the speed and efficiency of a large class of reductions. 
3. The Electrodeposition of Metals. — If hydrogen were evolved at a 
cathode at its reversible potential under all conditions no metal higher 
in the electromotive series than hydrogen could be deposited electro- 
lytically from an aqueous solution. Due to hydrogen overvoltage, 
however, it is possible to obtain deposits of these metals as high in the 
o 
\o 
J 
O 
o 
ZO 
CM3. m 
40 
.^^^^ o 
60 
FIG. 1 
series as zinc. The current efficiency of such a deposition depends, 
mainly, upon the relative magnitudes of the potential between the solu- 
tion and the metal, and the hydrogen overvoltage of the metal; the 
efficiency increasing, of course, with an increase of the overvoltage. 
The figures in table 2 indicate that the increase of hydrogen overvoltage 
produced by a decrease of pressure is accompanied, in the case of the 
deposition of zinc, by an increase in the efficiency of the deposition. 
These depositions were all made from the same solution, and at the 
same current density and temperature. 
TABLE 2 
Pressure, cms. Hg 76 13 12 6 
Efficiency, cm^ 75 81 92 97.5 
1 These Proceedings, May, 1919, also /. Amer. Chem. Soc, 41, 1919, (194). 
^ Trans. Amer. Electrochem. Soc, 32, 1917, (257). 
