diminished in the fame circumftances.

For this purpose I kept for some months a quart bottle full of each of these kinds of air; but as different quantities of inflammable air vary very much in this refpect, it is not improbable but that nitrous air may vary also.

From one trial that I made, I conclude that nitrous air may be kept in a bladder much better than most other kinds of air. The air to which I refer was kept about a fortnight in a bladder, through which the peculiar smell of the nitrous air was very fenfible for feveral days. In a day or two the bladder became red, and was much contracted in its dimenfions. The air within it had loft very little of its peculiar property of diminishing common air.

I did not endeavour to afcertain the exact quantity of nitrous air produced from given quantities of all the metals which yield it; but the few observations which I did make for this purpose I fhall recite in this place :

SECTION VII.

Of AIR infected with the FUMES of BURNING CHARCOAL.

Air infected with the fumes of burning charcoal is well known to be noxious; and the Honourable Mr. Cavendish favoured me with an account of fome experiments of his, in which a quantity of common air was reduced from 180 to 162 ounce measures, by paffing through a red-hot iron tube filled with the duft of charcoal. This diminution he afcribed to fuch a deftruction of common air as Dr. Hales imagined to be the confequence of burning. Mr. Cavendish alfo obferved, that there had been a generation of fixed air in this procefs, but that it was abforbed by fope leys. This experiment I alfo repeated, with a fmall variation of circumftances, and with nearly the fame refult.

Afterwards I endeavoured to ascertain, by what appears to me to be an easier and more certain method, in what manner air is affected with the fumes of charcoal, viz. by fufpending bits of charcoal within glafs veffels, filled to a certain height with water, and ftanding inverted

1

in another veffel of water, while I threw the focus of a burning mirror, or lens, upon them. In this manner I diminished a given quantity of air one fifth, which is nearly in the fame proportion with other diminutions of air.

If, instead of pure water, I used lime-water in this process, it never failed to become turbid by the precipitation of the lime, which could only be occafioned by fixed air, either discharged' from the charcoal, or depofited by the common air. At first I concluded that it came from the charcoal; but confidering that it is not probable that fixed air, confined in any fubftance, can bear fo great a degree of heat as is neceffary to make charcoal, without being wholly expelled; and that in other diminutions of common air by phlogiston only, there appears to be a depofition of fixed air, I have now no doubt but that, in this cafe alfo, it is fupplied from the fame fource.

This opinion is the more probable, from there being the fame precipitation of lime, in this process, with whatever degree of heat the charcoal had been made. If, however, the charcoal had not been made with a very confiderable degree of heat, there never failed to be a permanent addition of inflammable air produced; which agrees with what I obferved before, that,

in converting dry wood into charcoal, the greateft part is changed into inflammable air.

I have fometimes found, that charcoal which was made with the most intense heat of a smith's fire, which vitrified part of a common crucible in which the charcoal was confined, and which had been continued above half an hour, did not diminish the air in which the focus of a burning mirror was thrown upon it; a quantity of inflammable air equal to the diminution of the common air being generated in the process: whereas, at other times, I have not perceived that there was any generation of inflammable air, but a fimple diminution of common air, when the charcoal had been made with a much lefs degree of heat. This fubject deferves to be farther investigated.

To make the preceding experiment with ftill more accuracy, I repeated it in quick filver; when I perceived that there was a small increase of the quantity of air, probably from a generation of inflammable air. Thus it ftood without any alteration a whole night, and part of the following day; when lime-water, being admitted to it, it presently became turbid, and, after fome time, the whole quantity of air, which was about four ounce measures, was diminished one fifth, as before. In this cafe, I carefully weighed

K 2

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weighed the piece of charcoal, which was exactly two grains, and could not find that it was fenfibly diminished in weight by the ope

ration.

Air thus diminished by the fumes of burning charcoal not only extinguifhes flame, but is in the highest degree noxious to animals; it makes no effervescence with nitrous air, and is incapable of being diminished any farther by the fumes of more charcoal, by a mixture of iron filings and brimstone, or by any other caufe of the diminution of air that I am acquainted with.

This obfervation, which refpects all other kinds of diminished air, proves that Dr. Hales was mistaken in his notion of the abforption of air in thofe circumstances in which he observed it. For he supposed that the remainder was, in all cafes, of the fame nature with that which had been abforbed, and that the operation of the fame caufe would not have failed to produce a farther diminution; whereas all my obfervations fhew that air, which has once been fully dimi nished by any caufe whatever, is not only incapable of any farther diminution, either from the fame or from any other caufe, but that it has likewife acquired new properties, most remarkably different from those which it had before, and that they are, in a great measure, the

fame