_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 circumstances. 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 God 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 circumstance, and
to this, or rather to something always attending it, we say that the
appearance is _owing_. This circumstance 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 principal 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 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 _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 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.