I consider the properties of particular substances, it will be proper to mention those which are common to them all. But I shall first observe, that the term substance has no proper idea annexed to it, but is merely a convenience in speech; since we cannot speak of the properties of substances, such as hard, round, coloured, &c. &c. (which circumstances alone affect our senses, and thereby give proper ideas) without saying that they inhere in, or belong to, some thing, substance, or substratum. The terms being and person are also in the same predicament.
One property of all substances is extension, since they all occupy some portion of space.
The incapacity of any substance to change its place has been termed, though improperly, the vis inertiæ of matter. It is sufficient to say, that neither this, nor any other effect can be produced without a cause.
Infinite divisibility is a necessary property of all extended substance; and from this circumstance it will follow, that the smallest quantity of solid matter may be made to fill the largest space, and yet none of the pores shall exceed the smallest given magnitude; and consequently, that, for any thing we know to the contrary, all the bodies in the universe may be comprized in the smallest space.
Another property usually ascribed to all matter is impenetrability, or the necessary exclusion of any substance from the place occupied by another. But the only proof of impenetrability is the resistance that we find to our endeavours to put one substance into the place of another; and it is demonstrated by experiments, that this resistance is not occasioned by the actual contact of the substances, but by a power of repulsion acting at a real distance from their surfaces. It requires a considerable force to bring two solid substances into as near contact as the particles of the same substance; and that these are not in actual contact is evident, from their being capable of being brought nearer by cold; and this is most remarkable with respect to the heaviest, that is, the densest, of all substances, viz. the metals.
A more positive argument for the penetrability of matter is, that the particles of light, after entering the densest transparent substance, do not appear to meet with any obstruction to their progress till they come to the opposite side.
The powers of attraction and repulsion seem to be common 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.
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.
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.
