OXYGEN. Historical.-- Oxygen was rec ognized by its properties as far back as the 8th century among the Chinese who knew that the "active" component of the air combines with some metals, with sulphur and with char coal and that this active component could be obtained pure from saltpetre and certain other minerals. Leonardo da Vinci (1451-1519) was the first European to state that the air contains two gases, but it was not until 1774 that Joseph Priestley made the first pure sample of oxygen. It is true that Scheele, a Swedish apothecary, had made oxygen in 1771-72 from at least seven different substances, and that he had made quite an extensive study of its combination with various materials, but as his results were not printed until 1777, Priestley is generally con sidered the discoverer of oxygen. Many other chemists worked on air and the commonly known gases at that time, and these studies furnished the material on which Lavoisier, the great French philosopher and scientist, based his conclusions which may be said to form the foundation of modern chemistry. The name oxygen (meaning acid-torming) was given to the gas by Lavoisier who at that time thought it was an essential constituent of all acids.
Occurrence.— Oxygen is the most widely distributed element in nature and it has been estimated that it makes up nearly half of ter restrial matter. It forms approximately 21 per cent by volume of the atmosphere; it makes up eight-ninths by weight of all the water on the globe; more than three-fifths of the human body; nearly half of three of the chief con stituents of the earth's crust, namely, silicious rock, chalk and alumina. Many other minerals contain oxygen in considerable proportions. It is an essential constituent of all living organ isms, aside from its existence in the water of the tissues. It is absorbed by all animals dur ing respiration and is given off in the free state by growing vegetable organisms when exposed to sunlight.
Preparation.-- Pure oxygen is best pre pared in small quantities trom one of the many substances in which it occurs in chemical com bination.
I. Heat applied under the proper conditions causes oxygen to liberated from the follow ing compounds: oxides of mercury, silver, gold and platinum; peroxides of hydrogen, barium, lead and manganese; chlorates, nitrates, bi chromates, etc.. of potassium and other metals, Of chief interest are the following: 1. Ignition of nitre, which gives up only one-third of its oxygen. Priestley first ob tained impure oxygen in this manner in 1771.
2. Ignition of mercuric oxide, the method used by Priestley on 1 Aug. 1774, when he made the first pure oxygen.
3. Heating manganese' dioxide, formerly one of the cheapest methods of commercial prep aration of oxygen, and the one used by Scheele.
4. Heating potassium chlorate alone preferably with about one-eighth of its weight of manganese dioxide, or a little spongy plati num. Oxygen prepared in this way usually con tains chlorine which can be removed by caus tic soda solution. The manganese dioxide (pyrolusite) should be tested as the occasional adulteration of it with coal dust has been the cause of fatal accidents.
II. A convenient laboratory method for the preparation of pure oxygen is to treat aoxone (a convenient form of sodium peroxide) with water. This material is prepared in a handy form especially for this purpose.
HI. The electrolysis of water made slightly acid with sulphuric acid furnishes pure oxygen, which is liberated at the positive pole, and at the same time twice the volume of hydrogen is liberated at the negative pole.
Properties,— Oxygen is a colorless, taste less and odorless gas. It is 1.1056 times as heavy as air. Owing to the large number of elements that combine•with oxygen, it has been selected by the International Committee on Atomic Weights as the standard of comparison for all atomic weights, that of oxygen being 16 (according to this scale the atomic weight of hydrogen is 1.008).