Experiments and Observations on Different Kinds of Air

by Joseph Priestley

In considering this experiment, I soon perceived that this curious appearance must have been occasioned by the mixture of the nitrous and common air, and therefore that the white clouds must be nitrous ammoniac, formed by the acid of the nitrous air, set loose in the decomposition of it by common air, while the phlogiston, which must be another constituent part of nitrous air, entering the common air, is the cause of the diminution it suffers in this process; as it is the cause of a similar diminution, in a variety of other processes.

I would observe, that it is not peculiar to nitrous air to be a test of the fitness of air for respiration. Any other process by which air is diminished and made noxious answers the same purpose. Liver of sulphur for instance, the calcination of metals, or a mixture of iron filings and brimstone will do just the same thing; but the application of them is not so easy, or elegant, and the effect is not so soon perceived. In fact, it is phlogiston that is the test. If the air be so loaded with this principle that it can take no more, which is seen by its not being diminished in any of the processes above mentioned, it is noxious; and it is wholesome in proportion to the quantity of phlogiston that it is able to take.

This, I have no doubt, is the true theory of the diminution of common air by nitrous air, the redness of the appearance being nothing more than the usual colour of the fumes, of spirit of nitre, which is now disengaged from the superabundant phlogiston with which it was combined in the nitrous air, and ready to form another union with any thing that is at hand, and capable of it.

With the volatile alkali it forms nitrous ammoniac, water imbibes it like any other acid, even quicksilver is corroded by it; but this action being slow, the redness in this mixture of nitrous and common air continues much longer when the process is made in quicksilver, than when it is made in water, and the diminution, as I have also observed; is by no means so great.

I was confirmed in this opinion when I put a bit of volatile alkaline salt into the jar of quicksilver in which I made the mixture of nitrous and common air. In these circumstances, the vessel being previously filled with the alkaline fumes, the acid immediately joined them, formed the white clouds above mentioned, and the diminution proceeded almost as far as when the process was made in water. That it did not proceed quite so far, I attribute chiefly to the small quantity of calx formed by the slight solution of mercury with the acid fumes not being able to absorb all the fixed air that is precipitated from the common air by the phlogiston.

In part, also, it may be owing to the small quantify of surface in the quicksilver in the vessels that I made use of; in consequence of which the acid fumes could act upon it only in a slow succession, so that part of them, as well as of the fixed air, had an opportunity of forming another union with the diminished air.

This, as I have observed before, was so much the case when the process was made in quicksilver, without any volatile alkali, that when water was admitted to it, after some time, it was not capable of dissolving that union, tho' it would not have taken place if the process had been in water from the first.

In diversifying this experiment, I found that it appeared to very great advantage when I suspended a piece of volatile salt in the common air, previous to the admission of nitrous air to it, inclosing it in a bit of gauze, muslin, or a small net of wire. For, presently after the redness of the mixture begins to go off, the white cloud, like snow, begins to descend from the salt, as if a white powder was shaken out of the bag that contains it. This white cloud presently fills the whole vessel, and the appearance will last about five minutes.

If the salt be not put to the mixture of these two kinds of air till it has perfectly recovered its transparency, the effervescence being completely over, no white cloud will be formed; and, what is rather more remarkable, there is nothing of this appearance when the salt is put into the nitrous air itself. The reason of this must be, that the acid of the nitrous air has a nearer affinity with its phlogiston than with the volatile alkali; though the phlogiston having a nearer affinity with something in the common air, the acid being thereby set loose, will unite with the alkaline vapour, if it be at hand to unite with it.

There is also very little, if any white cloud formed upon holding a piece of the volatile salt within the mouth of a phial containing smoking spirit of nitre. Also when I threw the focus of a burning mirror upon some sal ammoniac in nitrous air, and filled the whole vessel with white fumes which arose from it, they were soon dispersed, and the air was neither diminished nor altered.