What did Cavendish describe in his paper "On Factitious Airs"?
Density of "inflammable air"
The process of photosynthesis
The properties of helium
The structure of the atom
Answer
Cavendish measured the density of "inflammable air" (which we now know to be hydrogen) and found that it was 1/14th the density of air. This led him to conclude that hydrogen was a very light gas.