I beleive those held battery oil.
The early batteries use a liquid electrolite that tended to evaporate.
The oil was poured on the surface to keep the electrolite from evaporating as fast.
When I am able to upload some pictures I'll show you the first ones they made with the oil still in them. The ones I have are the cork top variety with the Thomas A Edison signature across the top with 4OZ SPECIAL BATTERY OIL under that and THOMAS A EDISON inc. under that with GRANGE NJ USA under that.