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 constitute 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.