THE DISCOVERY OF GASES - Priestley 5#

Priestley

Joseph Priestley was born in 1733 at Fieldhead, England. He aquired a knowledge of Hebrew, Greek and Latin which, with his extensive theological learning, enabled him to meet on more than equal terms his more orthodox brethen.

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Figure 20. Joseph Priestley (1733 – 1804)

In 1772, Priestley published a long and important paper, Experiments and Observations on Different Kinds of Air. In this he describes a pneumatic through with a shelf for collecting gases over water, also the methods of collecting and manipulating gases and the preparation of several new gases. Nitrous air was obtained from copper, etc. and dilute nitric acid; phlogisticated air (nitrogen); nitrous vapour (nitrogen dioxide) from copper or bismuth and concentrated nitric acid; acid air (hydrochloric acid) which, as it was very soluble in water, was collected over mercury. Cavendish had already collected gases over water an mercury in 1766 and had discovered nitrogen before 1772.

By using the mercury trough Priestley was able to collect several gases soluble in water (he describes in his book, Experiments and Observations on Different Kinds of Air), such as alkaline air (ammonia, 1773 – 1774), vitriolic acid air (sulphur dioxide, November 1774) and fluor acid air (silicon fluoride, before November 1775).

In 1771, Priestley recognized that air which has been vitiated by puterfaction, by the breathing of animals or by burning of candles, is restored by growing green plants (mint, spinach or groundsel). In June and the following months of 1778, he found that aquatic plants growing in water containing dissolved fixed air (carbon dioxide) give off oxygen. He says, “the injury which is continually done to the atmosphere by the respiration of such a large animals ... is, in part of least, repaired by the vegetable creation”. Priestley did not at first realize the important part played by light in the change, now called photosynthesis. Senebier in 1782 showed that green plants, under the influence of light, convert fixed air into dephlogisticated air (oxygen).

Figure 21. A part of Priestley’s Laboratory

Priestley obtained oxygen on 1st August, 1774, at Calne by heating red oxide of mercury by a burning glass, and showed that :

1. This gas was practically insoluble in water

2. This gas supported the combustion of a candle in a dazzling manner.

Priestley had really obtained oxygen in 1771 by heating nitre, but he had confused it with air. In his paper on Observations on Different Kinds of Air, he says :”in one quantity (of air) which I got from saltpetre a candle not onlyburned, but the flame was increased and something was heard like a hissing, similar to the decrepitation of nitre in open air”.

Priestley assumed that a candle on burning gives out phlogiston and is extinguished in a closed vessel after a time because air becomes saturated with phlogiston. Ordinary air, therefore, supports combustion because it is only partially saturated with phlogiston and can absorb more of it. Substances burn in air with only a moderate flame, whereas in the new air the flame is vivid. Priestley, therefore, concluded that the new gas must contain little or no phlogiston and hence he called it dephlogisticated air. The gas left when bodies burnt out in ordinary air was named, for similar reason, phlogisticated air.

Phlogisticated Air (Nitrogen) = Air + φ (Scheele’s Foul Air)

Dephlogisticated Air (Oxygen) = Air – φ (Scheele’s Fire Air)

Early in 1775, Priestley found that a mouse lived twice as long in the new air as in the same confined volume of common air. He breathed it himself and fancied his “breast felt peculiarly light and easy for some time afterwards”. Hence, he recommended its use in medicine (Note : it is now used in the treatment of gas poisoning and pneumonia).

By his experiments, Priestley assumed that air contains one-fifth of its volume of dephlogisticated air (oxygen), which is an acurate result (Note : In the recent, the percentages of oxygen is almost 20 per cent). Scheele had found from one-fourth to one-third and Lavoisier, by various methods, found from one-sixth to one-fourth. Priestley was on the whole an accurate experimenter and his quantitative results are generally good.

Priestley also discovered nitrous oxide. He also obtained nitrogen dioxide by the action of concentrated nitric acid on copper or bismuth and by heating lead nitrate and noticed that it darkened in color on heating. By heating iron with a burning-glass in nitric oxide, he obtained half of the volume of nitrogen. He also obtained crystalline nitrososulphuric acid by the action of nitrogen dioxide on concentrated sulphuric acid.