He is generally credited with having discovered hydrogen, since he had described the density of 'inflammable air', which formed water on combustion, in a paper "On Factitious Airs" that appeared in 1766. Antoine Lavoisier later reproduced his experiment and gave the element its name.
Cavendish is also credited with one of the earliest accurate calculations of the mass of the earth. He used a torsion balance to measure the gravitational attraction between lead spheres in 1798, from which he calculated Newton's gravitational constant, 'G', which he used to calculate the earth's mass.
He was silent and solitary, viewed as somewhat eccentric, and formed no close personal relationships outside his family.
He left a large estate on his death which was used to endow the Cavendish Laboratory at Cambridge University in 1871.