[84] M: pp. 68, 70-71, 97, 129, 179-180, 311, 315, 317-335 Gilbert implied (M: p. 166), that a terrella does not rotate as Peregrinus said, due to resistance (M: p. 326), or due to the mutual nature of coition (M: p. 166); or even to the rotation of the earth (M: p. 332). However (M: p. 129), he also mentioned that a terrella would revolve by itself!
But before reaching this point in his argument, Gilbert digressed to classify the different kinds of attractions and motions which the elements produce. In particular, he distinguished electric attraction from magnetic coition, and pointed out the main features of electrical attraction. Since the resultant motions were different, the essential natures of electric and magnetic substances had to differ.
Gilbert introduced his treatment of motion by discussing the attraction of amber. All sufficiently light solids[85] and even liquids,[86] but not flame or air[87] are attracted by rubbed amber. Heat from friction,[88] but not from alien sources like the sun[89] or the flame,[90] produce this "affection." By the use of a detector modeled after the magnetic needle, which we would call an electroscope but which he called a "versorium,"[91] Gilbert was able to extend the list of substances that attract like amber.[92] These Gilbert called "electricae."[93]
[93] M: p. 78. The definition Gilbert gave of an electric in the glossary at the beginning of his treatise was not an experimental one: "Electricae, quae attrahunt eadem ratione ut electrum."
Possibly as a result of testing experimentally statements like that of St. Thomas, on the effect of garlic on a loadstone, Gilbert discovered that the interposition of even the slightest material (except a fluid like olive oil) would screen the attraction of electrics.[94] Hence the attraction is due to a material cause, and, since it is invisible, it is due to an effluvium.[95] It must be much rarer than air,[96] for if its density were that of air or greater, it would repel rather than attract.[97]
The source of the effluvia could be inferred from the properties of the electrics. Many but not all of the electrics are transparent, but all are firm and can be polished.[98] Since they retain the appearance and properties of a fluid in a firm solid mass,[99] Gilbert concluded that they derived their growth mostly from humors or were concretions of humors.[100] By friction, these humors are released and produce electrical attraction.[101]