Before revision: In the 19th century, it was common to use town gas to fill balloons; this was not as light as pure hydrogen gas, having about half the lifting power, but it was much cheaper and readily available.
After revision: In the 19th century, it was common to use manufactured town gas (coal gas) to fill balloons; this was not as light as pure hydrogen gas, having about half the lifting power, but it was much cheaper and readily available.