What properties would have caused early researchers to name hydrogen "inflammable air"

It ignited in air to form water

Explanation:

In the experiments of Cavendish, he discovered a gas lighter than air when he added certain metals to dilute acids. The gas when breathed is significantly different from air.

The gas is known to burn in air when it approaches a flame to form water hence the name 'inflammable air' was given to the gas. It was later called hydrogen which means 'water former'.

Cavendish noticed the production of a gas when a piece of Zn or Mg dropped in an acid. He can collect the produced gas which is colorless. The gas was like the air but Cavendish noticed by breathing that it is different from air. So, he called it "inflammable air" which is known today as hydrogen gas.