Experiments and Observations on Different Kinds of Air

by Joseph Priestley

§ 1. Speculations concerning the constituent principles of the different kinds of air, and the constitution and origin of the atmosphere, &c.

All the kinds of air that appear to me to be essentially distinct from each other are fixed air, acid and alkaline; for these, and another principle, called phlogiston, which I have not been able to exhibit in the form of air, and which has never yet been exhibited by itself in any form, seem to constitute all the kinds of air that I am acquainted with.

Acid air and phlogiston constitute an air which either extinguishes flame, or is itself inflammable, according, probably, to the quantity of phlogiston combined in it, or the mode of combination. When it extinguishes flame, it is probably so much charged with the phlogistic matter, as to take no more from a burning candle, which must, therefore, necessarily go out in it. When it is inflammable, it is probably so much charged with phlogiston, that the heat communicated by a burning candle makes it immediately separate itself from the other principle with which it was united, in which separation heat is produced, as in other cases of ignition; the action and reaction, which necessarily attends the separation of the constituent principles, exciting probably a vibratory motion in them.

Since inflammable, air, by agitation in water, first comes to lose its inflammability, so as to be fit for respiration, and even to admit a candle to burn in it, and then comes to extinguish a candle; it seems probable that water imbibes a great part of this extraordinary charge of phlogiston. And that water can be impregnated with phlogiston, is evident from many of my experiments, especially those in which metals were calcined over it.

Water having this affinity with phlogiston, it is probable that it always contains a considerable portion of it; which phlogiston having a stronger affinity with the acid air, which is perhaps the basis of common air, may by long agitation be communicated to it, so as to leave it over saturated, in consequence of which it will extinguish a candle.

It is possible, however, that inflammable air and air which extinguishes a candle may differ from one another in the mode of the combination of these two constituent principles, as well as in the proportional quantity of each; and by agitation in water, or long standing, that mode of combination may change. This we know to be the case with other substances, as with milk, from which, by standing only, cream is separated; which by agitation becomes butter. Also many substances, being at rest, putrefy, and thereby become quite different from what they were before. If this be the case with inflammable air, the water may imbibe either of the constituent parts, whenever any proportion of it is spontaneously separated from the rest; and should this ever be that phlogiston, with which air is but slightly overcharged, as by the burning of a candle, it will be recovered to a state in which a candle may burn in it again.

It will be observed, however, that it was only in one instance that I found that strong inflammable air, in its transition to a state in which it extinguishes a candle, would admit a candle to burn in it, and that was very faintly; that then the air was far from being pure, as appeared by the test of nitrous air; and that it was only from a particular quantity of inflammable air which I got from oak, and which had stood a long time in water, that I ever got air which was as pure as common air. Indeed, it is much more easy to account for the passing of inflammable air into a state in which it extinguishes candles, without any intermediate state, in which it will admit a candle to burn in it, than otherwise. This subject requires and deserves farther investigation. It will also be well worth while to examine what difference the agitation of air in natural or artificial sea-water will occasion.

Since acid air and phlogiston make inflammable air, and since inflammable air is convertible into air fit for respiration, it seems not to be improbable, that these two ingredients are the only essential principles of common air. For this change is produced by agitation in water only, without the addition of any fixed air, though this kind of air, like various other things of a foreign nature, may be combined with it.

Considering also what prodigious quantities of inflammable air are produced by the burning of small pieces of wood or pit-coal, it may not be improbable but that the volcanos, with which there are evident traces of almost the whole surface of the earth having been overspread, may have been the origin of our atmosphere, as well as (according to the opinion of some) of all the solid land.

The superfluous phlogiston of the air, in the state in which it issues from volcanos, may have been imbibed by the waters of the sea, which it is probable originally covered the surface of the earth, though part of it might have united with the acid vapour exhaled from the sea, and by this union have made a considerable and valuable addition to the common mass of air; and the remainder of this over-charge of phlogiston may have been imbibed by plants as soon as the earth was furnished with them.