On this day 251 years ago, on August 1, 1774 (June 24, 1774 in the lunar calendar), British chemist Priestley discovered oxygen while heating mercury oxide in his laboratory. On August 1, 1774, British chemist Priestley discovered oxygen while heating mercury oxide in his laboratory. Among the research achievements of gas chemistry, Priestley's most important discovery of oxygen. In 1774, he obtained a large convex lens (fire mirror) and began to study the various gases emitted by certain substances at the high temperature where the convex lens concentrates light. Among the substances he studied were "red precipitate" (mercury oxide) and "mercury ash", also known as mercury cinder, that is, (mercury oxide). Priestley placed the mercury oxide on the mercury surface in the glass bell jar, and used a fire mirror with a diameter of 30 cm and a focal length of 50 cm to concentrate sunlight on the mercury oxide. Soon it was discovered that the mercury oxide had been decomposed, releasing a gas that squeezed out the mercury inside the glass cover. He called this gas "dephlogistonized air". He collected this gas by drainage gas collection method and then studied its properties. It is found that candles will burn in this air, and the flame is very bright. Mice live normally in this gas, and they live about 4 times longer than in the same volume of ordinary air. He also tried it himself, and felt that this air made people breathe briskly and comfortably. He described the whole process of the experiment in detail. Comment: You must be good at discovering in a laboratory, otherwise no matter how many experiments you do, it will be futile!