We must now direct the attention of the reader to _oxygen gas_, the discovery of which was made by Dr. Priestley in the year 1774, and by him called _dephlogisticated air_; the most important discovery that was, perhaps, ever made in chemistry. When a metal is exposed to atmospheric air, at almost every temperature, it loses its metallic lustre, and acquires the form and appearance of an earthy substance. If this change be produced in a given quantity of air, the _oxidation_ can only be carried on to a certain degree; and on examining the air which remains, we shall find that it has lost the whole of its oxygen, and that nothing remains but nitrogen gas. What was formerly called the _calcination_ of metals is nothing but the process of their union with oxygen, which is now therefore properly called their _oxidation_. If charcoal be mixed with the metallic oxide, and a suitable heat be applied to the mixture, it will unite with the oxygen and form carbonic acid, which will fly off in the form of gas, while the metal will assume its metallic form. From this we learn that _oxygen_ is a part of atmospheric air, and that _nitrogen_ constitutes another portion of the same air. _Ammonia_ is a combination of nitrogen and hydrogen. _Combustion_, or the burning of any combustible body, cannot take place, at least under ordinary circumstances, without the presence of oxygen. _Nitrogen gas_, (called by its discoverers _azotic gas_), constitutes about three fourths of atmospheric air; the other fourth consists of oxygen, besides a small fraction of carbonic acid gas. Oxygen decomposes and destroys all fugitive colours. Oxygen is, besides, the basis of almost all the acids, and hence is one of the most universal agents in nature. _Hydrogen_, formerly called _inflammable air_, was discovered by Mr. Cavendish in 1767; it is called hydrogen, because it is one of the component parts of water; or, more properly, it is the base of water. It is obtained in the most pure state from the decomposition of water by means of metals, thus: pass one hundred parts of water through a red hot iron tube, a gun barrel for instance, fifteen parts of hydrogen gas will be produced, while the inside of the tube will be found converted into an oxide, and to have gained eighty five parts in weight. Again, when eighty five parts of oxygen gas are burned with fifteen of hydrogen gas, both gases vanish, and one hundred parts of water are the result. Hydrogen gas, when in a pure state, is about fifteen times lighter than atmospheric air; hence its use for inflating balloons. Hydrogen, if inhaled, destroys animal life; combined with _nitrogen_, it forms ammonia, or the _volatile alkali_, as we have before stated.