These early balloons used hydrogen. While atomic hydrogen is only one fourth the weight of atomic helium, hydrogen forms a diatomic molecule as a gas, and thus has a molecular weight of half of helium, reducing the lift advantage. Since hydrogen is explosive in mixtures with air of more than 4 percent, it is a great fire hazard and serious explosion hazard in large quantities. Helium being an inert gas does not form a diatomic molecule as a gas. Thus hydrogen is only half the weight of helium, not one fourth as you might suspect by looking it up on a periodic table. H2 = 2 * Atomic Weight 1; vs He = 1 * Atomic Weight 4.

Hydrogen is easily manufactured by several chemical reactions such as the action of hydrochloric acid on mossy zinc metal and the action of sodium hydroxide on aluminum metal pellets or punchings. This ability to make the gas on the spot has always been attractive. During the civil war special wagons with large wooden tanks full of acid were used to generate the inflation hydrogen required on the spot. 

The link even has lift tables  It's all about balloon lift with lighter than air gases.