322 
CHEMISTRY: MACINNES AND CONTIERI 
(e = electron). However, no chemical action will ensue unless another 
reaction, involving the absorption of the electrons liberated in Reac- 
tion 1, can also occur. In the absence of oxidizing agents the only- 
possible reaction is 
2H+ -\-2e = ^ (2) 
Any factor which tends to decrease the velocity of Reaction 2, i.e., 
which increases the overvoltage, will decrease the corrosion represented 
by Reaction 1 . Watts and Whipple^ have found that, contrary to state- 
ments in the chemical literature, a decrease of the external pressure 
will produce a decrease of corrosion of metals in acids. These workers, 
however, attribute the decreased corrosion to the absence of air in 
the solutions which were under reduced pressure. As it seemed prob- 
able to us that the effect is, largely at least, due to an increase in the 
overvoltage with decreased pressure, we repeated their experiments, 
taking care to exclude oxygen from the acid. Table 1, which contains 
TABLE 1 
METAL 
LOSS IN WEIGHT 
At 1 atmospheric 
pressure 
At ^5 atmospheric 
pressure 
Zn 
gram 
0.0927 
0.0797 
0.0018 
gram 
0.0594 
0.0469 
0.0018 
Fe 
Cd 
typical results for Zn, Fe, and Cd, shows that the effect of decreased 
pressure on the corrosion of the first two of these metals is in the direc- 
tion predicted. No bubbles of hydrogen were observed to leave the 
surface of the cadmium, so no change of overvoltage, and therefore no 
change of the corrosion, with the pressure is to be expected. 
2. Reduction, in Acid Solutions, by Metals. — On placing a strip of 
iron into a slightly acidified solution of FeCls the metal enters solution 
according to Reaction 1. The two ionic reactions that compete, so to 
speak, for the liberated electrons are 
Fe+++ + e = Fe+++ (3) 
and Reaction 2. Here we can predict that a decrease of pressure will, 
by increasing the overvoltage, reduce the velocity of Reaction 2, with 
the result that Reaction 3 will be favored. The experimental results 
of a series of measurements are plotted in figure 1, in which abscissas 
represent pressures and ordinates reduction in a given time. Due to 
