Before entering into a discussion of the question, which naturally arises at this point, as to how levity and gravity by their two possible ways of interaction - 'sulphurous' or 'saline' - determine the properties of so-called positive and negative electricity, we shall first study the third mode of generating electricity, namely, by electromagnetic induction. Along this way we shall arrive at a picture of the magnetic force which corresponds to the one already obtained of electricity. This will then lead us to a joint study of the nature of electric polarity and magnetic polarity.