In the 1700's, the field of chemistry was just starting to really distinguish itself from physics. Henry Cavendish was an English chemist and physicist who helped to pioneer this field, and is mainly known for discovering hydrogen as well as for the ambitious "Cavendish experiment," which gave an estimate of the density of the Earth. He was born in 1731 in France, the son of aristocrats. A well-known hermit, this eccentric scientist built secret staircases so that he could communicate with servants only through written notes, and was only spotted in public to attend scientific lectures or events.
This dedication to solitude allowed Henry Cavendish to obsessively pursue his true love, science. He carried out hundreds of chemistry experiments in his home laboratory. While only a few of these were put into print, the record of experiments has managed to survive. One of his most famous printed papers was the "1766 Three Papers Containing Experiments on Factitious Airs." This detailed a series of experiments that he conducted, including reactions between solids and liquids to create chemical reactions. One of the most important was a demonstration of just how hydrogen and carbon dioxide differed from the atmosphere at large.
The discovery of hydrogen, which Henry Cavendish called "inflammable air," is perhaps his most important contribution to the modern world of science. He discovered this in 1781, and how hydrogen differed from oxygen in the air. This work was also explored in greater detail by other scientists in operation at the same time, including Antoine Lavoisier and James Watt. Another discovery that he made was that there is a residue in decomposed air that cannot be broken down any further, which he called an "inert" gas. Later scientists identified this same inert gas as the element argon. Cavendish's work also helped lead to the discovery of nitric acid.
In 1798, Henry Cavendish published his estimate of the earth's density, which is now almost exactly what modern scientists have also deemed it to be. In many ways, he is now considered to be almost a century ahead of his time. He also started to pioneer the field of electricity, which was then taken further by scientists decades later. Although many of his works remained unpublished during his lifetime, the physicist James Clerk Maxwell published them a century later in 1879, and they proved to be extremely influential on a new generation of scientists. Cavendish's work remains highly regarded to this day.