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to all matter, and the component parts of all substances are kept in their places by the due balance of those opposite powers. If, by any means, the particles of any substance be removed beyond their sphere of mutual attraction, they repel one another, as those of water when it becomes steam.

      Of the different kinds of attraction, that of gravitation seems to extend to the greatest possible distance; but that which keeps together the parts of the same substance, thence called the attraction of cohesion, and the different kinds of chemical attractions, called affinities, only act at a small distance. Of the causes of these attractions we are entirely ignorant.

Of Aeriform Substances.

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      Aeriform substances, of which the air that we breathe is one, though invisible, are real substances, as appears by their excluding other substances.

      That the air has weight appears by actually weighing a vessel before and after it is exhausted of air by means of an air-pump (an instrument contrived for that purpose) by its bursting a bladder, and various other experiments.

      Air, being a fluid, presses in all directions, as in the experiment of the fountain in vacuo, and others.

      The weight of the air is the cause of the suspension of mercury in a barometer, and of the action of pumps. The weight of atmospherical air is to that of water in the proportion of about 1 to 800, so as to press with the weight of about fourteen pounds on every square inch of surface.

      Air, being an elastic fluid, is capable of occupying more or less space according to the pressure which it sustains, as appears by a bladder partially filled with air being expanded when the air is drawn from a receiver in which it is put, by means of the air-pump, and compressed in the condensing engine, an instrument the reverse of the air-pump.

      Air is necessary to the conveyance of sound, to the existence of flame, and to animal life.

LECTURE III.

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Of Atmospherical Air.

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      The first species of air that offers itself to our consideration is that of the atmosphere, which appears to consist of a mixture of two kinds of air, of different and opposite qualities, viz, dephlogisticated and phlogisticated, in the proportion of about one third of the former to two thirds of the latter. It is by means of the former of these two ingredients that it is capable of supporting flame and animal life.

      This composition of atmospherical air is proved by several substances absorbing the dephlogisticated air, and leaving the phlogisticated. All these processes have been termed phlogistic, because the effect is not produced but by substances supposed to contain phlogiston in a volatile state; and by the affinity between phlogiston and the dephlogisticated part of the air, the one is separated from the other. Of these processes are the calcination of metals, a mixture of iron-filings and sulphur, liver of sulphur, the burning of phosphorus, and the effluvia of flowers.

      In some cases, however, it is not so clear that any thing is emitted from the substance that produces this effect; for water deprived of all air will absorb the dephlogisticated part of the atmospherical in preference to the phlogisticated part.

      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.

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Of Dephlogisticated Air.

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      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.

      Dephlogisticated air is something heavier than atmospherical air, and the purity of it measured by mixing with it two equal quantities 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.

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      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 quantity 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.

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