fity with refpect to air, they being exactly plus and minus to each other.

The hypothefis maintained in this fection, viz. that atmospherical air confifts of the nitrous acid and earth, fuits exceedingly well with the facts relating to the production of nitre, for it is never generated but in the open air, and by expofing to it fuch kinds of earth as are known to have an affinity with the nitrous acid; fo that by their union common nitre may be formed.

Hitherto it has been fuppofed by chymifts, that this nitrous acid, by which common nitre is formed, exifts in the atmosphere as an extraneous fubftance, like water, and a variety of other fubftances, which float in it, in the form of effluvia; but fince there is no place in which nitre may not be made, it may, I think, with more probability be fuppofed, according to my hypothefis, that nitre is formed by a real decompofition of the air itself, the bases that are presented to it having, in such circumstances, a nearer affinity with the spirit of nitre than that kind of earth with which it is united in the atmosphere.

My theory alfo fupplies an eafy folution of what has always been a great difficulty with chymifts, with refpect to the detonation of

nitre. The question is, what becomes of the nitrous acid in this cafe?. The general, I be lieve the univerfal, opinion now is, that it is deftroyed; that is, that the acid is properly decompofed, and refolved into its original elements, which Stahl fuppofed to be earth and water. On the other hand, I fuppofe that, though the common properties of the acid, as combined with water, disappear, it is only in confequence of its combination with fome earthy or inflammable matter, with which it forms fome of the many fpecies of air, into the compofition of which this wonderful acid enters. It may be common air, it may be dephlogisticated air, or it may be nitrous air, or fome of the other kinds, of which an account will be given in a fubfequent fection. That it fhould really be the nitrous acid, though so much disguised by its union with earthy, or other matters, will not appear extraordinary to any person who fhall confider how little the acid of vitriol is apparent in common fulphur.

With respect to mercurius calcinatus, and red load, their red colour favours the fuppofition of their having extracted fpirit of nitre from the air.

An

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SECTION IV.

A more particular Account of fome Proceffes for the Production of Dephlogisticated Air.

I cannot promise those of my readers, whose object is nothing more than general information, much pleasure from the perufal of this section, as it will confift, for the most part, of a dry detail of proceffes, for procuring dephlogisticated air; but as they all appeared neceffary, in my investigation of the fubject, I doubt not but an attention to them will be of use to fuch as are difpofed to purfue the inquiry themselves. I might have contented myself with giving a general idea of the refult of fuch experiments; but that would have been to mix my own opi nions with facts, in fuch a manner that the reader would not have been able to separate them. At prefent, if I fhould be mistaken in any of my opinions, the reader, having before him all the facts on which thofe opinions were grounded, will be able to rectify the mistake, and prevent the error from spreading.

Having feen fufficient reafon to conclude that refpirable air confifts of nitrous acid and earth, my object, in all this course of experi

ments,

ments, was fimply to find what kind of earth was most proper for this purpose, or which had the most aptness to form this peculiar union with the nitrous acid. Upon the whole, I think it will appear that the metallic earths, if they be free from phlogiston, are the most proper, and next to them the calcareous earths but that a very great difference in the production of this kind of air depends upon a variety of circumstances in which the experiments are made.

I have obferved that red lead, without any addition, yields dephlogisticated air by heat. To give some idea of the differences in the refults, from what is, to appearance, the fame preparation, and of the confequence of adding fpirit of nitre to the red lead, I muft inform my reader, that having weighed two halfounces of red lead, taken from the fame parcel, I put one of them, without any addition, into the gun-barrel, and with a very brisk fire (which is generally a confiderable advantage for the production of air) I got no more than three ounce-measures, and it was very little. better than common air.

The fecond half-ounce I moistened with a very diluted spirit of nitre; and when it was dried and pounded, I put it into the same gunbarrel;

barrel; and, in the fame circumstances with the former, I got from it about three pints of air, the first part of which was fo far dephlogifticated, that two measures of it, and five of nitrous air, occupied the space of two meafures only; of the fecond quantity, two meafures were not increased by the addition of feven measures of nitrous air. This was the

pureft air that I had then feen. The last produce was almost all pure fixed air, being not at all affected by nitrous air, extinguishing a candle, and precipitating lime in lime-water. It was, indeed, a little of a nitrous nature; for it diminished common air in a small degree, an effect which I attribute to the phlogiston coming from the iron.

A remarkable difference in the quantity of the produce of this kind of air, as I hinted juft now (and as I have obferved in a former publication, in the produce of inflammable air) depends upon the fuddennefs with which the fame degree of heat is applied. The following must be reckoned a remarkable fact of this kind, and it was made with as much care as I could poffibly apply. From an ounce of red lead, by a sudden and brisk heat, I got above two quarts of air, a great part of which was fixed air, and the reft was about twice as good as common air; and immediately after,

putting