The formal forces of a loadstone emanate in all directions from it,[167] but there is a bound to it that Gilbert called the "orbis virtutis."[168] The shape of this "orbis virtutis" is determined by the shape of the stone.[169] This insensible effusion is analogous to the spreading of light that reveals its presence only by opaque bodies.[170] Similarly, the magnetic forms are effused from the stone,[171] and can only reveal their presence by coition with another loadstone or by "awakening" magnetic bodies within the "orbis virtutis."[172] Unmagnetized iron that comes within the "orbis virtutis" is altered, and the magnetic virtue renews a form that is already potentially in the iron.[173] The formal energy is drawn not only from the stone but from the iron.[174] This is not generation, or alteration in the sense of a new impressed quality, but alteration in the sense of the entelechy or the activation of a form potentially present.[175] Those bodies magnetized by coming within the "orbis virtutis" have in turn an efflux of their own.[176] Iron can also receive verticity directly from the earth without the intervention of an ordinary loadstone.[177] Such verticity can be expelled and annulled by the presence of another loadstone.[178] [167] M: pp. 121, 123, 124, 304, 305, 306, 307, 309. [168] 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. [169] M: pp. 124, 150, 151. [170] M: pp. 123, 307. [171] M: pp. 304-307. See also p. 310, where it is stated that the sun and earth could awaken souls. [172] M: pp. 101, 110, 112, 123, 148, 149, 304, 305. This awakening of the iron within the "orbis virtutis" is comparable (pp. 216, 350) to the birth of a child under the influence of the stars. [173] M: pp. 110, 111, 112, 189, 216, 217. See also footnote 36. [174] M: p. 106. [175] M: pp. 106, 109, 110. [176] M: pp. 113, 114. [177] M: pp. 190, 192, 210-216. [178] M: p. 209.