Lucy Mack Smith

THE MAKERS OF SCIENCE 781

and contact with many great men, though he still made his living in the ministry. It was believed in those days that air was an elementary — thing in fact it had so been classed from the days of the Greek philosophers. In the course of his researches, however, Priestley came to the conclusion that air is not elementary, but a mixture of substances. This thought led him to the discovery by which he will always be remembered. On the first of August, 1774, he obtained in his studies a gas that he had not before encountered and with very surprising properties. A lighted candle placed in this gas burned with a remarkably vigorous flame, and a piece of red-hot wood sparkled in it and was consumed very fast. Finally he found that mice and insects lived nearly five times as long in this gas as in an equal volume of air. To complete the test, Priestly breathed some of the gas and he relates that he fancied his breathing peculiarly light and easy for some time afterwards. He soon came to the conclusion that this gas is found in the atmosphere, and is the ingredient which enables air to support combustion and animal life. The gas thus discovered by Priestley was, of course, oxygen, which forms about one-fifth of the atmosphere. It is now the knowledge of everyone that when anything burns it unites with the oxygen in the air. In breathing, the oxygen is taken up by the blood and the impurities burned and the carbonic acid gas which is formed is breathed out. It is the oxygen mixed with the water that enables fish to live in water. Frequently, in serious cases of sickness, pure oxygen is given the patient to stimulate the bodily activities. The discovery of the element oxygen by Priestley and of the nature of combustion by Lavoisier, made possible a new understanding of chemistry and physiology. Though these two men discovered what seems to us most elementary truths, it is by the possession of these truths that much scientific progress has been made possible. Priestley made many other scientific discoveries of note; in fact, to the day of his death he gave some time to the study of science. Priestley was of a democratic turn of mind. The republican form of government appealed to him. This was not wholly pleasing to the English people. When the Bastile in Paris fell, in 1791, a fanatical mob, knowing Priestley's sympathy for the J«Vr.Tnch revolutionists, attacked and burned his house and church, and destroyed his papers and scientific instruments. Priestley escaped, but three years later removed to the United States, where he settled at Northumberland, Pennsylvania.