As the purity of atmospherical air, or its fitness for respiration, depends upon the proportion of the dephlogisticated air that it contains, any of the above-mentioned processes will suffice to determine it. The more any given quantity of air is diminished by any of them, the purer it was before the diminution. But this effect is produced the most quickly by a mixture of nitrous air, or firing inflammable air in it, being almost instantaneous. In order to measure the purity of air, it is convenient to take more of the nitrous or inflammable air than is necessary to saturate the dephlogisticated air it contains. Equal quantities of each best answer the purpose. Supposing a given quantity of atmospherical air to be mixed with an equal quantity of nitrous air, and the residuum to be 1.1 measure, the proportion of dephlogisticated and phlogisticated air in it may be found by the following arithmetical operation, it being here taken for granted that one measure of pure dephlogisticated air will saturate two measures of pure nitrous air. 2.0 viz. one of each. 1.1 the residuum. ----- 3)0.9 the quantity that has disappeared. 0.3 the dephlogisticated air contained in the measure of the air examined. And this substracted from 1 leaves .7 for the proportion of phlogisticated air in it. LECTURE IV. _Of Dephlogisticated Air._ Dephlogisticated air, which is one of the component parts of atmospherical air, is a principal 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 quantity of atmospherical air. In respiration, part of this air, passing 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.