above-mentioned for the purpose, which, with a fmall burning lens, and an uncertain fun, is not a little troublesome. But all that I obferved for fome time was, that I ftood the best chance of fucceeding when I warmed the veffel in which the mixture was made, and agitated the air during the effervescence.

Finding, at length, that, with the fame preparations and attentions, I got the fame ap. pearance from a mixture of nitrous and common air in the fame trough of water, I concluded that it could not depend upon any thing peculiar to the precipitate of the copper contained in the paper from which the air was procured, as I had at first imagined, but upon what was common to it, and pure nitrous air.

Afterwards, having, (with a view to obferve whether any crystals would be formed by the union of volatile alkali, and nitrous air, fimilar to those formed by it and fixed air, as described by Mr. Smeth in his Differtation on fixed Air) opened the mouth of a phial which was half filled with a volatile alkaline liquor, in a jar of nitrous air (in the manner defcribed p. 11. fig. 4.) I had an appearance which perfectly explained the preceding. All that part of the phial which was above the liquor, and which

contained

contained common air, was filled with beautiful white clouds, as if fome fine white powder had been inftantly thrown into it, and some of these clouds rose within the jar of nitrous air. This appearance continued about a minute, and then intirely disappeared, the air becoming transpa

rent

Withdrawing the phial, and expofing it to the commón air, it there also became turbid, and foon after the tranfparency returned. Introducing it again into the nitrous air, the clouds appeared as before. In this manner the white fumes, and transparency, succeeded each other alternately, as often as I chose to repeat the experiment. and would no doubt have continued till the air in the jar had been thoroughly diluted with common air. These appearances were the fame with any fubftance that contained volatile alkali, fluid or folid.

When, instead of the small phial, I used a large and tall glafs jar, this appearance was truly fine and ftriking, especially when the water in the trough was very transparent. For I had only to put the fmalleft drop of a volatile alkaline liquor, or the smallest bit of the folid falt, into the jar, and the moment that the mouth of it was opened in a jar of nitrous air, the white clouds above mentioned began to

be

be formed at the mouth, and prefently defcended to the bottom, fo as to fill the whole, were it ever fo large, as with fine fnow.

In confidering this experiment, I foon perceived that this curious appearance must have been occafioned 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, fet loofe in the decompofition of it by common air, while the phlogifton, which must be another constituent part of nitrous air, entering the common air, is the cause of the diminution it fuffers in this procefs; as it is the caufe of a fimilar diminution, in a variety of other proceffes.

I would obferve, that it is not peculiar to nitrous air to be a teft of the fitness of air for refpiration. Any other procefs by which air is diminished and made noxious anfwers the fame purpose. Liver of fulphur for inftance, 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 fo eafy, or elegant, and the effect is not fo foon perceived. In fact, it is phlogiston that is the teft. If the air be fo loaded with this principle that it can take no more, which is feen by its not being diminished in any of the proceffes

above mentioned, it is noxious; and it is wholefome 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 fpirit of nitre, which is now difengaged from the fuperabundant phlogifton 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 quickfilver is corroded by it; but this action being flow, the redness in this mixture of nitrous and common air continues much longer when the process is made in quick filver, than when it is made in water, and the diminution, as I have also obferved, is by no means fo great.

I was confirmed in this opinion when I put a bit of volatile alkaline falt into the jar of quickfilver in which I made the mixture of nitrous and common air. In these circumstances, the veffel being previously filled with the alkaline fumes, the acid immediately joined them, formed the white clouds above mentioned, P

and

and the diminution proceeded almost as far as when the process was made in water. That it did not proceed quite fo far, I attribute chiefly to the small quantity of calx formed by the flight folution 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, alfo, it may be owing to the small quantity of furface in the quickfilver in the veffels that I made ufe of; in confequence of which the acid fumes could act upon it only in a flow fucceffion, fo 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 procefs was made in quickfilver, without any volatile alkali, that when water was admitted to it after, fome time, it was not capable of diffolving that union, though it would not have taken place if the process had been in water from the first.

In diverfifying this experiment, I found that it appeared to very great advantage when I fufpended a piece of volatile falt in the common air, previous to the admiffion of nitrous