PART I.

Experiments and Obfervations made in, and before the year 1772.

Ν

IN writing upon the fubject of different kinds of air, I find myself at a lofs for proper terms, by which to distinguish them, those which have hitherto obtained being by no means fufficiently characteristic, or diftinct. The only terms in common use are, fixed air, mephitic, and inflammable. The laft, indeed, fufficiently characterizes and diftinguishes that kind of air which takes fire, and explodes on the approach of flame; but it might have been termed fixed with as much propriety as that to which Dr. Black and others have given that denomination, fince it is originally part of fome folid fubftance, and exists in an unelastic state.

All these newly discovered kinds of air may also be called factitious; and if, with others, wẹ ufe the term fixable, it is ftill obvious to remark, that it is applicable to them all; fince they are all capable of being imbibed by fome fubftance or other, and confequently of being fixed in them, after they have been in an elaftic state.

The term mephitic is equally applicable to what is called fixed air, to that which is inflammable, and to many other kinds; fince they are equally noxious, when breathed by animals. Rather, however, than either introduce new terms, or change the fignification of old ones, I fhall use the term fixed air, in the sense in which it is now commonly used, and distinguish the other kinds by their properties, or fome other periphrafis. I fhall be under a neceffity, however, of giving names to those kinds of air, to which no names had been given by others, as nitrous, acid, and alkaline.

SECT.

SECTION I.

Of FIXED AIR.

It was in confequence of living for fome time in the neighbourhood of a public brewery, that I was induced to make experiments on fixed air, of which there is always a large body, ready formed, upon the furface of the fermenting liquor, generally about nine inches, or a foot in depth, within which any kind of fubftance may be very conveniently placed, and though, in these circumstances, the fixed air must be continually mixing with the common air, and is therefore far from being perfectly pure, yet there is a conftant fresh supply from the fermenting liquor, and it is pure enough for many purposes.

A perfon, who is quite a ftranger to the properties of this kind of air, would be agreeably amufed with extinguifhing lighted candles, or chips of wood in it, as it lies upon the furface of the fermenting liquor; for the smoke readily unites with this kind of air, probably by means of the water which it contains; fo that very little or none of the smoke will efcape into the open air, which is incumbent upon it. It is remarkable, that the upper furface of this fmoke,

fmoke, floating in the fixed air, is smooth, and well defined; whereas the lower furface is exceedingly ragged, several parts hanging down to a confiderable distance within the body of the fixed air, and sometimes in the form of balls, connected to the upper ftratum by slender threads, as if they were fufpended. The fmoke is also apt to form itself into broad flakes, parallel to the furface of the liquor, and at different diftances from it, exactly like clouds. These appearances will fometimes continue above an hour, with very little variation. When this fixed air is very strong, the fmoke of a fmall quantity of gunpowder fired in it will be wholly retained by it, no part escaping into the common air.

Making an agitation in this air, the furface. of it, (which still continues to be exactly defined) is thrown into the form of waves, which it is very amusing to look upon; and if, by this agitation, any of the fixed air be thrown. over the fide of the veffel, the fmoke, which is mixed with it, will fall to the ground, as if it was fo much water, the fixed air being heavier than common air.

The red part of burning wood was extinguished in this air, but I could not perceive that a red-hot poker was fooner cooled in it.

Fixed air does not inftantly mix with common air. Indeed if it did, it could not be caught upon the furface of the fermenting liquor. A candle put under a large receiver, and immediately plunged very deep below the furface of the fixed air, will burn fome time. But veffels with the fmalleft orifices, hanging with their mouths downwards in the fixed air, will in time have the common air, which they contain, perfectly mixed with it. When the fermenting liquor is contained in veffels clofe covered up, the fixed air, on removing the cover, readily affects the common air which is contiguous to it; so that, candles held at a confiderable distance above the surface will inftantly go out. I have been told by the workmen, that this will fometimes be the cafe, when the candles are held two feet above the mouth of the veffel.

Fixed air unites with the smoke of rofin, fulphur, and other electrical fubftances, as well as with the vapour of water; and yet, by holding the wire of a charged phial among these fumes, I could not make any electrical atmofphere, which furprized me a good deal, as there was a large body of this fmoke, and it was fo confined, that it could not escape me.

I also held fome oil of vitriol in a glass veffel within the fixed air, and by plunging a