This Day In History: 1774-08-01

Unitarian preacher Joseph Priestley identified  a gas which he called – “dephlogisticated air” later known as oxygen. Priestley found that mercury heated in air became coated with “red rust of mercury,” which, when heated separately, was converted back to mercury with “air” given off.  Studying this “air” given off, he observed that candles burned very brightly in it. Also, a mouse in a sealed vessel with it could breathe it much longer than ordinary air. A strong believer in the phlogiston theory, Priestley considered it to be “air from which the phlogiston had been removed.” Further experiments convinced him that ordinary air is one fifth dephlogisticated air, the rest considered by him to be phlogiston.