The green color here is due to nickel oxide which is present in the stone to the amount sometimes of one per cent.
The Edison storage cell (Figs. 269 and 270) has plates of iron and nickel oxide.
But the nickel oxide will readily give up its excess of oxygen, and the iron will receive it back freely if permitted.
Before going further we must consider another oxide—namely, nickel oxide.
The ignited substance is nickel oxide (NiO), and contains 78.67 per cent.
The battery is said to be discharging as long as the iron is receiving oxygen from the nickel oxide.
An ammoniacal solution of nickel oxide was prepared by dissolving the precipitated hydrated oxide in concentrated ammonia.