Note 168 — AND HIS PREDECESSORS

Gilbert defined the orbis virtutis in the glossary at the beginning of his treatise as, "... totum illud spatium, per quod quaevis magnetis virtus extenditur." This is the core of the difference between electric and magnetic forces. The substantial form of an electric could not be "effused," but was "imprisoned" in matter (as the Neoplatonic soul in the human body); while the primary form of a magnet did not require a material carrier and its effusion was similar to the propagation of a species in light.