LECTURE IV.

_Of Dephlogisticated Air._

Dephlogisticated air, which is one of the component parts of atmospherical air, is a princ.i.p.al element in the composition of acids, and may be extracted by means of heat from many substances which contain them, especially the nitrous and vitriolic; as from nitre, red precipitate, the vitriols, and turbith mineral, and also from these two acids themselves, exposed to a red heat in an earthen tube. This kind of air is also contained in several substances which had attracted it from the atmosphere, as from precipitate _per se_, _minium_, & _manganese_.

Dephlogisticated air is likewise produced by the action of light upon green vegetables; and this seems to be the chief means employed by nature to preserve the purity of the atmosphere.

It is this ingredient in atmospheric air that enables it to support combustion and animal life. By means of it the most intense heat may be produced, and in the purest of it animals will live nearly five times as long as in an equal quant.i.ty of atmospherical air.

In respiration, part of this air, pa.s.sing the membrane of the lungs, unites with the blood, and imparts to it its florid colour, while the remainder, uniting with phlogiston exhaled from the venous blood, forms fixed air. It is dephlogisticated air combined with water that enables fishes to live in it.

Dephlogisticated air is something heavier than atmospherical air, and the purity of it measured by mixing with it two equal quant.i.ties of nitrous or inflammable air, deducing the residuum after the diminution from the three measures employed, and dividing the remainder by 3, as in the process for common air.

_Of Phlogisticated Air._

The other ingredient in the composition of atmospherical air is phlogisticated air. It is procured by extracting the dephlogisticated part of the common air, as by the calcination of metals, &c. &c. by dissolving animal substances in nitrous acid, and also by the union of phlogiston with nitrous air, as by heating iron in it, and by a mixture of iron-filings and sulphur.

Phlogisticated air extinguishes a candle, is entirely unfit for respiration, and is something lighter than common air. It is not capable of decomposition, except by exploding it together with a superabundance of dephlogisticated air, and a quant.i.ty of inflammable air, or by taking the electric spark repeatedly in a mixture of it and dephlogisticated air. In these cases nitrous acid is formed.

LECTURE V.

_Of Inflammable Air._

Inflammable air is procured from all combustible substances by means of heat and water, and from several of the metals, especially iron, zink, and tin, by the vitriolic and marine acids.

From oils and spirit of wine it is procured by the electric spark. By the same means also alkaline air is converted into it.

That which is procured from metals, especially by steam, is the purest and the lightest, about ten times lighter than common air; in consequence of which, if a sufficient quant.i.ty be confined in a light covering, it is possible to make it carry up heavy weights.

When it is procured from animal or vegetable substances, it is of a heavier kind, and burns with a lambent flame, of various colours, according to the circ.u.mstances.

Calces of metals heated in inflammable air are revived, and the air absorbed; and since all the metals are revived in the same inflammable air, the principle of metallization, or _phlogiston_, appears to be the same in them all.

Pure inflammable air seems to consist of phlogiston and water, and the lambent kinds to be the same thing, with the addition of some oily vapour diffused through it.

LECTURE VI.

_Of Nitrous Air._

Nitrous air is procured by dissolving most of the metals, especially iron, mercury, and copper, in the nitrous acid; but that from mercury seems to be the purest. Nitrous air produced from copper contains a mixture of phlogisticated air. Some nitrous air is also obtained from the solution of all vegetable substances in nitrous acid; whereas animal substances in the same process yield chiefly phlogisticated air: but in both these cases there is a mixture of fixed air.

This species of air is likewise produced by impregnating water with nitrous vapour. This process continues to have this effect after the water becomes blue, but ceases when it turns green; there not then, probably, being a sufficient proportion of water. Nitrous air is likewise produced by volatile alkali pa.s.sing over red hot manganese, or green vitriol, when they are yielding dephlogisticated air. This shews that dephlogisticated air is one ingredient in the composition of nitrous air, and the same thing appears by pyrophorus burning in it. On the contrary, when nitrous air is made to pa.s.s over red-hot iron, volatile alkali is produced.

Nitrous air is completely decomposed by a mixture of about half its bulk of dephlogisticated air, and the produce is nitrous acid. And as nitrous acid is likewise formed by the union of inflammable and dephlogisticated air, one princ.i.p.al ingredient in nitrous air must be common to it and inflammable air, or phlogiston. This air is likewise decomposed by dephlogisticated nitrous acid, which by this means becomes phlogisticated. It is also decomposed by a solution of green vitriol, which by this means becomes black, and when exposed to the air, or heated, emits nitrous air, and recovers its former colour. These decompositions of nitrous air seem to be effected by depriving it of phlogiston, and thereby reducing it to the phlogisticated air originally contained in it.

This kind of air is diminished to about one fourth of its bulk by a mixture of iron filings and brimstone, or by heating iron in it, or calcining other metals in it, when the remainder is phlogisticated air.

All that iron gets in this process is an addition of weight, which appears to be water, but it loses its phlogiston, so that nitrous air seems to contain more phlogiston, and less water than phlogisticated air.

Nitrous air and dephlogisticated air will act upon one another through a bladder, but in this case there remains about one-fourth of the bulk of nitrous air, and that is phlogisticated air; so that in this case there seems to be a conversion of nitrous air into phlogisticated air without any addition of phlogiston.

Nitrous air is decomposed by pyrophorus, and by agitation in olive oil, which becomes coagulated by the process. It is also absorbed by spirit of turpentine, by ether, by spirit of wine, and alkaline liquors.

It is imbibed by charcoal, and both that air which is afterwards expelled from it by heat, and that which remains unabsorbed, is phlogisticated air.

Nitrous air resists putrefaction, but is diminished by the animal substances exposed to it to about a fourth of its bulk, and becomes phlogisticated air. It is likewise fatal to plants, and particularly to insects.

When nitrous air is long exposed to iron, it is diminished and brought into a state in which a candle will burn in it, though no animal can breathe it. But this peculiar modification of nitrous air, called _dephlogisticated nitrous air_, is produced with the greatest certainty by dissolving iron in spirit of nitre saturated with copper, impregnating water with this air, and then expelling it from the water by heat. If bits of earthen ware be heated in this dephlogisticated nitrous air, a great proportion of it becomes permanent air, not miscible with water, and nearly as pure as common air, so that the principle of _heat_ seems to be wanting to const.i.tute it permanent air.

LECTURE VII.

_Of Fixed Air._

Having considered the properties of those kinds of air which are not readily absorbed by water, and therefore may be confined by it, I proceed to those which _are_ absorbed by it, and which require to be confined by mercury. There are two kinds, however, in a middle state between these, being absorbed by water, but not very readily; a considerable time, or agitation, being necessary for that purpose. The first of these is _fixed air_.

This kind of air is obtained in the purest state by dissolving marble, lime-stone, and other kinds of mild calcareous earth in any acid. It is also obtained by the burning, or the putrefaction, of both animal and vegetable substances, but with a mixture of both phlogisticated and inflammable air. Fixed air is also produced by heating together iron filings and red precipitate; the former of which would alone yield inflammable air, and the latter dephlogisticated. Fixed air is therefore a combination of these two kinds of air.

Another fact which proves the same thing is, that if charcoal of copper be heated in dephlogisticated air, almost the whole of it will be converted into fixed air. On the same principle fixed air is produced when iron, and other inflammable substances, are burned in dephlogisticated air, and also when minium, and other substances containing dephlogisticated air, are heated in inflammable air.

That water is an essential part of fixed air is proved by an experiment upon _terra ponderosa aerata_, which yields fixed air when it is dissolved in an acid, but not by mere heat. If steam, however, be admitted to it in that state, it will yield as much fixed air as when it is dissolved in an acid.

Water absorbs something more than its own bulk of fixed air, and then becomes a proper acid. Iron dissolved in this water makes it a proper chalybeate; as without iron it is of the same nature with Pyrmont or Seltzer water, which by this means may be made artificially.

Ice will not imbibe this air, and therefore freezing expels it from water.

Fixed air extinguishes flame, and is fatal to animals breathing in it.

Also water impregnated with this air is fatal to fishes, and highly injurious to plants. But water thus impregnated will prevent, in a great measure, the putrefaction of animal substances.

Fixed air thrown into the intestines, by way of glyster, has been found to give relief in some cases of putrid disease.

_Of Hepatic Air._

Another species of air absorbed by water, but not instantly, is termed _hepatic air_, being produced by the solution of liver of sulphur, or of sulphurated iron, in vitriolic or marine acid.