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From ancient writings it appears that the Chinese, already in the eighth century, recognised that a substance, on burning, combined with one of the constituents of the air; it was also realised that this constituent of the atmosphere was present in water, and that it could be obtained in a pure condition by heating certain minerals. In Europe it was not until the middle of the seventeenth century that the atmosphere was regarded as a mixture of which one of the ingredients played an important part in combustion, respiration, and the change in colour of the blood. It was understood by Hooke (1665) and Mayow (1674) that saltpetre contains a substance of somewhat similar properties, but although the observation that saltpetre, when heated to decomposition, gives a vigorous evolution of gas was made only a little later, the actual discovery of oxygen was delayed until the next century, when the experimental methods first introduced by Mayow in 1674 for the collection of gases began to bear fruit. The gas was first prepared and recognised as a new substance by the Swedish chemist Scheele about the year 1771 as the result, amongst other methods, of heating red mercuric oxide or "mercurius calcinatus per se" by concentrating the sun's rays upon it with a lens. He termed the gas empyreal or fire air, and also showed that the same gas could be obtained from the yellow oxide produced by precipitation of a mercuric salt from aqueous solution on addition of an alkali. Unfortunately for himself these results were not published for some four years after their discovery, and in the meantime, namely on 1st August 1774, the English chemist Priestley, independently discovered oxygen, likewise by heating mercuric oxide. He communicated his results to Lavoisier in Paris in October of the same year, and shortly afterwards his discovery received general publication.

Theory of Phlogiston

In order to appreciate the enormous influence which the discovery of oxygen was destined to exert upon the further progress of chemistry, it is necessary to gain some idea of the views then prevalent as to the nature of combustion.

It is undoubtedly a fact that neither Scheele nor Priestley realised the important part played in combustion processes by the gas they had discovered.

By gradual modification the ancient Aristotelean idea of fire, as one of the four " elements " of nature, had merged into the assumption that all substances capable of burning contained a common combustible constituent or " principle." For several centuries sulphur appears to have been regarded as this principle, and its presence was postulated in all metals capable of being burnt or calcined in air. Becher, however, in 1669 took exception to this latter view, maintaining that sulphur owed its combustibility to the fact of its containing a large amount of combustible principle, but that sulphur itself was not that principle. He therefore gave the name terra pinguis or " oily earth " to the last named, and explained the calcination of metals by heating in air as due to the expulsion of terra pinguis. What, precisely, this terra pinguis might be, whether of a material or non-material nature, Becher did not say. Perhaps he regarded it as of a spiritual nature, like flame itself, and somewhat defying conventional definition. His views were accepted and amplified by Stahl, who, c. 1697, introduced the word phlogiston to denote the active principle producing fire. Like Becher, Stahl hesitated to define exactly the nature of his phlogiston. It corresponded to the terra pinguis of Becher and the sulphur of the earlier chemists. Any substance that would burn was regarded as being rich in phlogiston, and carbon was considered to be nearly pure phlogiston.

When metals are calcined in air, oxides are usually produced. This was explained by Stahl on the supposition that the metal, on being heated, parted with its phlogiston, leaving a residue of calx.

In the light of this idea a metallic calx or oxide was of simpler composition than the metal itself. Thus

metal = calx + phlogiston.

Further, reducing agents such as charcoal were substances which, being rich in phlogiston, could restore this to the metallic oxide and so regenerate the metal. In accordance with these views, Priestley named the gas which he obtained from mercuric oxide, dephlogisticated air as an expression of the readiness with which substances burned in or imparted phlogiston to it; nitrogen, on the other hand, which appeared incapable of supporting combustion, was regarded as being already saturated with phlogiston, and was termed by Priestley phlogisticated air.

The theory of phlogiston was, during the eighteenth century, exceedingly popular amongst chemists, despite the fact that it was full of anomalies. For example, if phlogiston were a material substance it is evident, from the equation given above, that a metal must weigh more than its calx. If phlogiston were non-material, the metal and calx would possess equal weight. But Jean Rey had already, in 1630, shown that lead and tin increase in weight when calcined, and Bay en, in 1774, found that on heating mercury calx to a sufficiently high temperature, metallic mercury is obtained, accompanied by a diminution in weight. Both of these facts are in direct opposition to the phlogistic theory.

Again, Stahl himself was not unaware of the fact that carbon would not burn in the absence of air, although, as mentioned above, he regarded it as almost pure phlogiston. The explanation offered was that phlogiston could not leave a substance unless it had somewhere to go. The air, however, could act as a sponge and absorb the phlogiston, and thus enable combustion to proceed. Such an explanation is, of course, inadequate in the case of the calcination of metals since no account is taken of the reduction in volume that invariably ensues.

It was reserved for Lavoisier to discover the true cause of combustion. Having satisfied himself that metals do increase in weight upon calcination, he definitely proved that this is due to their combining with Priestley's dephlogisticated air, and was thus led to discard the idea of a special principle of combustibility such as phlogiston. A new name was therefore necessary for Priestley's gas, and Lavoisier first called it " eminently pure air," but later oxygen (French oxygene), in his belief that the element was an essential constituent of all acids. The German name Sauerstoff embodies the same idea. Although subsequent research has demonstrated the inaccuracy of this assumption, the names have retained their popularity.
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