of its own strength, nor as if it injfclcci iniu llu- cthi-r 

 any physical substance; hut rather the dormant 

 power of the one is awakened 1)\ (he other's without 

 expenditure." In addition, tiie potency can be 

 passed only to the other."' 1-inally they both ha\e 

 the same history: 



We see both the finest magnet and iron ore visited as it 

 were by the same ills and disea.ses, acting in the same way 

 and with the same indications, preserved by the same 

 remedies and protective measures, and so retaining their 

 properties . . . they are both impaired by the action 

 of acrid iicinids as though by poison"'' . . . each is saved from 

 impairment by being kept in the scrapings of the other. 

 [So] . . . form, essence and appearance are one. " 



Any difference between the loadstone proper and 

 the iron proper is due to a difference in the actual 

 power of the magnetic \ irtue: " "Weak loadstones are 

 those disfigured witti dross metallic humors and with 

 foreign earth admi.xiures, [hence one may conclude] 

 they are further removed from the mother earth and 

 are more degenerate.'' 



(.ilbert's second induction was that they are ''true 

 and intimate parts of the globe,'' '" that is, that they 

 are piece of the "materia prima" of all we see about 

 us. For they "seem to contain within themselves 

 the potency of the earth's core and of its inmost 

 viscera.'' "" Whence, in Gilbert's philosophy, the 

 earthy matter of the elements was not passive or 

 inert *° as it was in .Aristotle's, but already had the 

 magnetic powers of loadstone. Being endowed with 

 properties, it was, in peripatetic terms, a simple body. 



If these pieces of earth i3ro]X'r, before decay, are 

 loadstones, then one may pass to the next induction 

 that the earth itself is a loadstone.'*' Conversely, a 

 terrella has all the properties of the earth: *" "Every 

 separate fragment of the earth exhibits in indubitable 

 experiments the whole impetus of magnetic matter; 

 in its various inovements it follows the terrestial globe 

 and the common princi]:)le of motion."*^ 



'< M: p. 62. 



" M: p. 63. 



'« M: p. 60. 



" M: pp. 19, 21, 43, 53, 61, 63, 184. 



™ M: p. 61. 



'» M: pp. 66, 67. 



*" M: p. 69. Gilbert is confusing Aristotelian matter and an 

 element. He includes cold and dry, wiili formless and inert! 

 Sec also Maier, op. cit. (footnote 17). 



«' M: p. 63; bk. 1, ch. 17. 



82 M: pp. 67, 181-183, 235-240, 281-289, 313-314. 



'^ M: p. 71. See also pp. 314 and 331. It is not clear, at 

 this point, whether he believed a "properly balanced" terrella 

 would be a perpetuum mobile. 



The next induction that Gilbert made was that as 

 the magnet possesses vcrticity and turns towards 

 the poles, so the loadstone-earth possesses a verticity 

 and turns on an axis fixed in direction.*'' He could 

 now discuss the motions of a loadstone in general, in 

 terms of its nature, just as an .\ristotelian di.scussed the 

 motion of the elements in terms of their nature. 



But before reaching this point in his argument, 

 Gilbert digressed to classify the difierenl kinds of 

 attractions and motions which the elements produce. 

 In partii iilar, he distinguished electric attraction froin 

 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 cliH'er. 



Ciilbert introduced his treatment of motion by dis- 

 cussing the attraction of amber. .Ml sufficiently 

 light solids "^ and even liquids,^" but not fiame or air *' 

 are attracted by rubbed amber. Heat from friction,** 

 but not from alien sources like the sun *" or the flame,*' 

 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 "versor- 

 ium," " Gilbert was able to extend the list of sub- 

 stances that attract like amber. '^ These Gilbert called 

 "electricae."' "^ 



Possibly as a result of testing experimentally state- 

 ments like that of St. Thomas, on the effect of garlic 

 on a loadstone, Gilbert discovered that the inter- 

 position of even the slightest material (except a fluid 

 like olive oil) would screen the attraction of elec- 

 trics.'^ Hence the attraction is due to a material 

 cause, and, since it is invisible, it is due to an efllu- 

 vium.^'' It inust be much rarer than air,''*" for if its 



*< 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 

 Percgrinus 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 men- 

 tioned that a terrella would revolve by itself! 



*5 M: pp. 78, 82, 84, 86. 



M M: pp. 78, 89, 91. 



8' M: pp. 89, 95. 



88 M: pp. 83, 86. 



8' M: pp. 81, 86, 87. 



«» M: pp. 80, 81, 86, 87. 



»' M: p. 79. 



M M: pp. 77-78, 79. 



»' M: p. 78. The d<-finition Gilbert gave of an electric in the 

 glossary at the beginning of his treatise was not an experimental 

 one: "Electricae, quae attrahunt eadem ratione ul electrum." 



9< M: pp. 86, 91, 135. « M: pp. 96, 135. 



«« M: p. 89. 



132 



BULLETIN 218: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY .-\ND TECHNOLOGY 



