f the active material. If a battery is
repeatedly over-discharged, this results in the positives shedding. In
the negatives, the spongy lead is puffed out, resulting in the
condition known as "bulged negatives" as illustrated in Fig 122.

2. Buckling. As a plate grid is bent out of shape, the active
material, especially the peroxide, breaks loose from the grid, since
the peroxide cannot bend as much as the grids. This occurs in the
negatives also, though not to such an extent as in the positives.

If the plates are buckled to such an extent that the element will not
go back into the jar, the positives should be discarded. If the
positives are buckled, the negatives will be also, but not to the
extent that the positives are.

In the case of the positives, there is no remedy, and the plates
should be discarded. The negatives, however, may be fully charged, and
then straightened, and the active material forced back flush with the
grids by pressings, as described in Chapter 15.

Impurities

Impurities may be divided into two general classes. The first class
includes those which do not attack the separators or grids, but merely
cause internal self-discharge. The second class includes those which
attack the grids or separators.

1. Impurities Which Merely Cause Self-discharge. This includes metals
other than lead. If these metals are in solution in the electrolyte,
they deposit on the negative plate, during charge, in their ordinary
metallic state, and form small cells with the spongy lead. These small
cells discharge as soon as the charging circuit is opened, and some of
the lead is changed to lead sulphate. This, of course, causes a loss
in capacity. Free hydrogen is given off by this local discharge, and
so much of it is at times given off that the hydrogen bubbles give the
electrolyte a milky appearance.

Silver, gold, and platinum are the most active in forming small local
cells. These metals form local cells which have comparatively high
voltages, and which take away a considerable portion of the energy of
a cell. Platinum is especially active, and a small amount of platinum
will prevent a negative plate from taking a charge. Gradually,
however, the spongy lead covers up the foreign metal and prevents it
from forming local cells.

Iron also forms local cells which rob the cell of a considerable
portion of its capacity. This may be brought into the cell by impure
acid or water. Iron remains in solution in t