Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry. Joseph Priestley
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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 circumstances 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 glass 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 æther, 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 quantities 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 principal 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 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