1383 
reason whatever why the lead should be scattered. If it is, however, 
assumed that the hydrogen, before it escapes as gas, penetrates into 
the lead down to a certain depth, then with the formation of gas 
bubbles in the lead, the metal can scatter. 
But this conception is not tenable withont any further hypothesis 
either. If namely the hydrogen separated in the form in which it 
escapes later on, so as molecules H,, it would be strange that the 
hydrogen does not escape at once, but tirst enters into the lead down 
to a certain depth. and does not escape until after this. We shall, 
therefore have to assume that the hydrogen separates in another 
form than that in which it afterwards escapes, and it is natural to 
suppose that this will be in the form of hydrogen atoms originating 
through the direct discharge of the hydrogen ions. 
These hydrogen atoms will combine to molecules H, with a certain 
velocity, and these will escape in gaseous form. When the atomic 
hydrogen is soluble in the metal, the hydrogen atoms will penetrate 
into the metal to a certain depth and combine there to molecules H,. 
If this molecular hydrogen is less soluble in the metal, the hydrogen 
will again try to escape from the metal. A kind of explosion takes 
place in the metal in consequence of which the metal is scattered 
in the solution in finely divided state. 
For the scattering of a metal in consequence of primary hydrogen 
generation the following conditions must therefore be satisfied. 
1. The hydrogen atoms formed by electrolysis must combine to 
molecules H, with a restricted velocity. 
2. The hydrogen atoms must be soluble in the metal, and there- 
fore with a certain velocity diffuse into the metal in which they 
combine to molecules H, with a limited velocity. 
3. The metal must probably not possess a too great hardness, so 
that the tension which arises in consequence of an excess of hydrogen 
in the metal is also able to break up the metal. 
The first condition includes that the hydrogen generation at the 
metal requires a certain overvoltage. For hydrogen can only be 
generated at the equilibrium potential of the hydrogen when the 
combination of 2H to H, proceeds so quickly that there is continually 
equilibrium between atomic and molecular hydrogen. If the concen- 
tration of the hydrogen atoms is greater than corresponds with the equili- 
brium, the potential of the electrode is more negative than the potential 
of equilibrium. Apart, therefore, from other causes for the overvoltage 
the slow combination of 2H to H, gives rise to a certain excess of 
potential difference. Different metals and also different electrolytes 
