Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry Part 2

LECTURE I.

_The Introduction._

The object of experimental philosophy is the knowledge of nature in general, or more strictly, that of the properties of natural substances, and of the changes of those properties in different circ.u.mstances. This knowledge can only be attained by _experiment_, or _observation_; as that clay is capable of becoming hard by means of fire, and thereby being made into bricks, and that by the same means lime-stone can be converted into quick-lime, and by the addition of water and sand, make mortar. It is by observation also that we discover that stones and other heavy bodies fall to the ground, and that a magnet will attract iron. In other words, experimental philosophy is an investigation of the wisdom of G.o.d in the works and _the laws of nature_, so that it is one of the greatest objects to the mind of man, and opens a field of inquiry which has no bounds; every advance we make suggesting new doubts and subjects of farther inquiry.

The uniformity we discover in the properties of natural substances enables us to lay down general rules, or principles, which, being invariable, we call the laws of nature; and by our knowledge of these laws we are able to predict, and at our own pleasure to produce, particular results, and this is the source of all the powers of man. It is the direction we acquire of the powers of nature; so that, as Lord Bacon observed, _knowledge is power_.

All arts and manufactures are derived from science. Thus the doctrine of _mechanics_ is an application of the law of gravitation. Every thing we are capable of doing by means of the steam-engine is derived from our knowledge of the properties of water in steam; all the great effects of gunpowder we owe to our knowledge of the composition, and chemical properties, of that substance.

Every new appearance in nature is preceded by some new circ.u.mstance, and to this, or rather to something always attending it, we say that the appearance is _owing_. This circ.u.mstance we therefore call the _cause_, and the new appearance the _effect_ of that cause. Thus we say that the union of phlogiston to a particular kind of earth is the cause of its becoming a metal.

It is one of the princ.i.p.al rules of philosophizing to admit no more causes than are necessary to account for the effects. Thus, if the power of gravity, by which heavy bodies fall to the earth, be sufficient to retain the planets in their orbits, we are authorized to reject the _Cartesian Vortices_. In other words, we must make no more general propositions than are necessary to comprehend all the particulars contained in them. Thus, after having observed that iron consists of a particular kind of earth united to phlogiston, and that it is soluble in acids; and that the same is true of all other metallic substances, we say, universally, that all metals consist of a peculiar earth and phlogiston, and that they are all soluble in some acid.

Of the circ.u.mstances which occasion a change in the properties of bodies, some are the addition of what are properly called _substances_, or things that are the objects of our senses, being _visible_, _tangible_, or having _weight_, &c. Thus the addition of an acid changes an alkali into a neutral salt. But other changes are occasioned either by a change of texture in the substance itself, or the addition of something that is not the object of any of our senses. Thus, a piece of steel becomes a magnet by the touch of another magnet, and a drop of gla.s.s acquires the property of flying asunder by a small fracture, in consequence of falling when red hot into cold water. Such also, in the opinion of some, is the difference between hot and cold substances.

Till the nature of the cause be ascertained, it is convenient to make use of the term _principle_, as including both of the above-mentioned causes of the change of properties in bodies. Thus, whatever be the real cause of gravity, or of inflammability, we may speak of the _principle of gravity_, or of _inflammability_; whether, with Newton, we suppose gravity to be occasioned by a fluid pervading the whole universe, which he termed _aether_, and whether inflammability be caused by the presence of a real substance called phlogiston, or not. In this manner we use the letters _x_ and _y_ to denote unknown quant.i.ties in algebra.

When changes are made in substances by the addition of other substances, they make what is called a _chemical union_; and in this case the properties of the compound cannot with any certainty be deduced from those of the component parts, but must be ascertained by fresh experiments. Thus, from the specific gravities, or the degrees of fusibility, of two metals, those of the compound cannot be predicted.

Neither water nor acid of vitriol will separately dissolve iron, so as to produce inflammable air, but both together will do it. However, the properties of similar compounds are similar to one another. Thus, all metals dissolved in acids are precipitated by mild alkalis. This chemical union of two substances we ascribe to a certain _elective attraction_, or _affinity_ that subsists between them, in consequence of which they unite with one another whenever a proper opportunity offers, in preference to those substances to which they were before united. Thus the vitriolic acid, having a stronger affinity with the vegetable alkali which is the basis of nitre, will unite with that alkali, and with it form another compound, called _vitriolated tartar_, while the acid of nitre, being detached from its base, is collected separately.

When two substances compose one liquid, and a third, which has a stronger affinity with either of the two parts than they have with each other, is added to them, it will unite with that part, and take its place in the solution, while the other will in many cases be precipitated, and may be collected. Thus the earth of alum is precipitated from a solution of alum by salt of tartar. This is the case of _simple affinity_.

When both the substances are compounds, the component parts of which have a weaker affinity with each other than they have with those of the other compound, two new combinations are formed, and this is called a case of _double affinity_. Thus when phlogisticated alkali is poured into a solution of green vitriol, the acid of the vitriol unites with the alkali, while the phlogiston joining the calx of iron makes Prussian blue.

All nature lying open to our investigation, we must consider the different parts in some order. But it is not very material which we adopt, because, begin where we will, the properties of the substances we first treat of will be connected with those which must be particularly considered afterwards, the changes in one substance being occasioned by its union with another. It will be impossible, for example, to explain the properties of metals without considering the _acids_, because by their union with acids very important changes are made in their properties.

There have been three princ.i.p.al methods of arranging natural substances.

One is according to the _three kingdoms_, as they are called, into which they have been distributed, viz. the _mineral_, the _vegetable_, and the _animal_. Another is according to the _elements_ which enter into their composition, and a third according to the _form_ in which they are usually found, viz. _aerial_, _fluid_, or _solid_. Upon the whole this last appears to me to be the most convenient, especially as it is easy to intermix general observations concerning the other divisions when they are particularly wanted. As there will be frequent occasion to speak of the component and elementary parts of all substances, I shall here observe, that, according to the latest observations, the following appear to be the elements which compose all natural substances, viz.

_dephlogisticated air_, or the _acidifying principle_; _phlogiston_, or the _alkaline principle_; the different _earths_, and the principles of _heat_, _light_, and _electricity_. Besides these, there are the following principles which have not been proved to be substances, viz.

_attraction_, _repulsion_, and _magnetism_. By the help of these principles we are able, according to the present state of natural knowledge, to explain all the appearances that have yet occurred to us.

LECTURE II.

_Of the Properties of all Matter._

Before 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 circ.u.mstances 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 s.p.a.ce.

The incapacity of any substance to change its place has been termed, though improperly, the _vis inertiae_ 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 circ.u.mstance it will follow, that the smallest quant.i.ty of solid matter may be made to fill the largest s.p.a.ce, 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 s.p.a.ce.

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 s.p.a.ce 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 quant.i.ty 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 quant.i.ties of each best answer the purpose. Supposing a given quant.i.ty of atmospherical air to be mixed with an equal quant.i.ty 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 quant.i.ty 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.