On August 1, 1774, he conducted his most famous experiment. Using a 12-inch-wide glass “burning lens,” he focused sunlight on a lump of reddish mercuric oxide. This was placed in an inverted glass container, which then was placed in a pool of mercury.
The gas emitted, he found, was “five or six times as good as common air.” In tests after that one, it caused a flame to burn intensely and kept a mouse alive for about four times as long as a similar quantity of air . We know now that that is because the oxygen was purer in that sample than normal atmospheric air, which contains more carbon dioxide, nitrogen, and other gases.