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kaline air is red, as it is in common inflammable air.

Though alkaline air be inflammable, it appeared, by the following experiment, to be heavier than the common inflammable air, as well as to contract no union with it. Into a veffel containing a quantity of inflammable air, I put half as much alkaline air, and then about the fame quantity of acid air. Thefe immediately formed a white cloud, but it did not rife within the space that was occupied by the inflammable air; fo that this latter had kept its place above the alkaline air, and had not mixed with it.

That alkaline air is lighter than acid air is evident from the appearances that attend the mixture, which are indeed very beautiful. When acid air is introduced into a veffel containing alkaline air, the white cloud which they form appears at the bottom only, and ascends gradually. But when the alkaline air is put to the acid, the whole becomes immediately cloudy, quite to the top of the veffel.

In the last place, I fhall obferve that alkaline air, as well as acid, diffolves ice as fast as a hot fire can do it. This was tried when both the kinds of air, and every inftrument made use of

in the experiment, had been exposed to a pretty intense frost several hours. In both cafes, also, the water into which the ice was melted diffolved more ice, to a confiderable quantity.

SECTION II.

Of COMMON AIR diminished and made noxious by various processes.

It will have been obferved that, in the first publication of my papers, I confined myself chiefly to the narration of the new facts which I had discovered, barely mentioning any hypothefis that occurred to me, and never seeming to lay much ftrefs upon them. The reafon why I was fo much upon my guard in this respect, was, left in confequence of attaching myfelf to any hypothefis too foon, the fuccefs of my future inquiries might be obstructed. But fubfequent experiments having thrown great light upon the preceding ones, and having confirmed the few conjectures I then advanced, I may now venture to speak of my hypothefes with a little lefs diffidence. Still, however, I fhall be ready to relinquish any notions I may now entertain, if new facts should hereafter арpear not to favour them.

!

In a great variety of cafes I have obferved that there is a remarkable diminution of common, or respirable air, in proportion to which it is always rendered unfit for refpiration, indifpofed to effervefce with nitrous air, and incapable of farther diminution from any other caufe. The circumstances which produce this effect I had then obferved to be the burning of candles, the refpiration of animals, the putrefaction of vegetables or animal fubstances, the effervescence of iron filings and brimstone, the calcination of. metals, the fumes of charcoal, the effluvia of paint made of white-lead, and oil, and a mixture of nitrous air.

All these proceffes, I observed, agree in this one circumstance, and I believe in no other, that the principle which the chemifts call phlogifton is fet loofe; and therefore I concluded that the diminution of the air was, in fome way or other, the confequence of the air becoming overcharged with phlogiston*, and that water, and growing vegetables, tend to restore this air

to

* On this account, if it was thought convenient to introduce a new term (or rather make a new application of a term already in ufe among chemifts) it might not be amifs to call air that has been diminished, and made noxious by any of the proceffes above mentioned, or others fimilar to them, by the common appellation of phlogifticated air;

and,

to a state fit for refpiration, by imbibing the fuperfluous phlogifton. Several experiments which I have fince made tend to confirm this fuppofition.

Common air, I find, is diminished, and rendered noxious by liver of fulphur, which the chemifts fay exhales phlogiston, and nothing elfe. The diminution in this cafe was one fifth of the whole, and afterwards, as in other fimilar cafes, it made no effervefcence with nitrous air.

I found also, after Dr. Hales, that air is diminished by Homberg's pyrophorus.

The fame effect is produced by firing gunpowder in air. This I tried by firing the gunpowder in a receiver half exhausted, by which the air was rather more injured than it would have been by candles burning in it.

Air is diminished by a cement made with one half common coarfe turpentine and half beeswax. This was the result of a very casual ob

and, if it was neceffary, the particular process by which it was phlogisticated might be added; as common air phlogifticated by charcoal, air phlogifticated by the calcination of metals, nitrous air phlogisticated with the liver of fulphur, &c. .

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fervation. Having, in an air-pump of Mr. Smeaton's conftruction, closed that end of the fyphon-gage, which is exposed to the outward air, with this cement (which I knew would make it perfectly air-light) inftead of fealing it hermetically; I obferved that, in a course of time, the quickfilver in that leg kept continually rifing, fo that the measures I marked upon it were of no ufe to me; and when I opened that end of the tube, and closed it again, the fame confequence always took place. At length, fufpecting that this effect must have arisen from the bit of cement diminishing the air to which it was exposed, I covered all the inside of a glass tube with it, and one end of it being quite closed with the cement, I fet it perpendicular, with its open end immersed in a bason of quickfilver; and was prefently fatisfied, that my conjecture was well founded: for, in a few days, the quickfilver rofe fo much within the tube, that the air in the infide appeared to be diminished about one fixth.

To change this air I filled the tube with quickfilver, and pouring it out again, I replaced the tube in its former fituation; when the air was diminished again, but not fo faft as before. The fame lining of cement diminished the air a third time. How long it will retain this power I cannot tell. This cement had

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