fixed air wants, to make it common air; and, for any thing I yet know, this may be the cafe, though I am ignorant of the method of combining them, and when I calcined a quantity of lead in fixed air, in the manner which will be defcribed hereafter, it did not feem to have been less soluble in water than it was before.

SECTION II.

Of AIR in which a CANDLE, or BRIMSTONE, has burned out.

It is well known that flame cannot fubfift long without change of air, fo that the common air is neceffary to it, except in the cafe of fubftances, into the compofition of which nitre enters, for these will burn in vacuo, in fixed air, and even under water, as is evident in fome rockets, which are made for this purpofe. The quantity of air which even a small flame requires to keep it burning is prodigious. It is generally faid, that an ordinary candle confumes, as it is called, about a gallon in a minute. Confidering this amazing confumption of air, by fires of all kinds, volcanos, &c. it becomes a great object of philofophical inquiry, to af scertain what change is made in the conftitution of the air by flame, and to discover what provi

fion there is in nature for remedying the injury which the atmofphere receives by this means. Some of the following experiments will, perhaps, be thought to throw light upon the subject.

The diminution of the quantity of air in which a candle, or brimstone, has burned out, is various; But I imagine that, at a medium, it may be about one fifteenth, or one fixteenth of the whole; which is one third as much as by animal or vegetable fubftances putrefying in it, by the calcination of metals, or by any of the other causes of the complete diminution of air, which will be mentioned hereafter.

I have sometimes thought, that flame difposes the common air to depofit the fixed air it contains; for if any lime-water be expofed to it, it immediately becomes turbid. This is the case when wax candles, tallow candles, chips of wood, fpirit of wine, ether, and every other fubftance, which I have yet tried, except brimstone, is burned in a clofe glafs veffel, ftanding in lime-water. This precipitation of fixed air (if this be the cafe) may be owing to fomething emitted from the burning bodies, which has a ftronger affinity with the other conflituent parts of the atmosphere*.

The fuppofition mentioned in this and other paffages of the first part of this publication, viz. that the diminution

of

If brimftone be burned in the fame circumftances, the lime-water continues transparent, but still there may have been the fame precipitation of the fixed part of the air; but that uniting with the lime and the vitriolic acid, it forms a felenetic falt, which is foluble in water. Having evaporated a quantity of water thus impregnated, by burning brimstone a great number of times over it, a whitish powder remained, which had an acid taste; but repeating the experiment with a quicker evaporation, the powder had no acidity, but was very much like chalk. The burning of brimstone but once over a quantity of lime-water, will affect it in fuch a manner, that breathing into it will not make it turbid, which otherwise it always prefently does.

Dr. Hales fuppofed, that by burning brimftone repeatedly in the fame quantity of air, the diminution would continue without end. But this I have frequently tried, and not found to be the cafe.

Indeed, when the ignition has been imperfect in the firft inftance, a fecond firing of the fame fubftance will increase the effect of the firft, &c. but this progrefs foon ceases.

of common air, by this and other proceffes is, in part at leaft, owing to the precipitation of the fixed air from it, the reader will find confirmed by the experiments and obfervations in the fecond part.

In

In many cases of the diminution of air, the effect is not immediately apparent, even when it ftands in water; for fometimes the bulk of air will not be much reduced, till it has paffed feveral times through a quantity of water, which has thereby a better opportunity of absorbing that part of the air, which had not been perfectly detached from the reft. I have fometimes found a very great reduction of a mass of air, in consequence of paffing but once through cold water. If the air has ftood in quickfilver, the diminution is generally inconfiderable, till it has undergone this operation, there not being any fubftance expofed to the air that could absorb any part of it.

I could not find any confiderable alteration in the fpecific gravity of the air, in which candles, or brimstone, had burned out. I am fatisfied, however, that it is not heavier than common air, which must have been manifest, if fo great a diminution of the quantity had been owing, as Dr: Hales and others fuppofed, to the elasticity of the whole mafs being impaired. After making feveral trials for this purpofe, I concluded that air, thus diminifhed in bulk, is rather lighter than common air, which favours the fuppofition of the fixed, or heavier part of the common air, having been precipitated.

An animal will live nearly, if not quite as long, in air in which candles have burned out, as in common air. This fact furprized me very greatly, having imagined that what is called the confumption of air, by flame, or refpiration, to have been of the fame nature, and in the fame degree; but I have fince found, that this fact has been obferved by many perfons, and even fo early as by Mr. Boyle. I have alfo obferved, that air, in which brimstone has burned, is not in the least injurious to animals, after the fumes, which at first make it very cloudy, have intirely fubfided.

I must in this place, admonish my reader not to confound the fimple burning of brimstone, or of matches (i. e. bits of wood dipped in it), and the burning of brimftone with a burning mirror, or any foreign heat. The effect of the former is nothing more than that of any other flame, or ignited vapour, which will not burn, unless the air with which it is furrounded be in a very pure ftate, and which is therefore extinguished when the air begins to be much vitiated. Lighted brimftone, therefore reduces the air to the same state as lighted wood. But the focus of a burning mirror thrown for a fufficient time, either upon brimstone, or wood, after it has ceafed to burn of its own accord, and has become charcoal, will have a much greater