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.