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._ 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. _Of Atmospherical Air._ 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.