density were that of air or greater, it would repel 

 rather than attract."^ 



The source of the eflluvia could be inferred from 

 the properties of the electrics. Many but not all of 

 the electrics are transparent, but all are firm and can 

 lie polished."" Since they retain the appearance and 

 properties of a fluid in a firm solid mass,"" Gilbert 

 concluded that they derived their »rowth mostly 

 from humors or were concretions of humors.'"" Hy 

 friction, these humors arc released and produce 

 electrical attraction.'"' 



This humoric source of the effluvia was substan- 

 tiated by Gilbert in a number of ways. Electrics lose 

 their power of electrical attraction upon being 

 heated, and this is because the humor has been driven 

 off.'"- Bodies that arc about equally constituted of 

 earth and humor, or that are mostly earth, have 

 been degraded and do not show electrical attrac- 

 tion.'"^ Bodies like pearls and metals, since they are 

 shiny and so must be made of humors, must also emit 

 an effluvium upon being rubbed, but it is a thick and 

 vaporous one without any attractive powers.'"* 

 Damp weather and moist air can weaken or even 

 prevent electrical attraction, for it impedes the efflux 

 of the humor at the source and accordingly diminishes 

 the attraction.'"' Charged bodies retain their powers 

 longer in the sun than in the shade, for in the shade 

 the effluvia are condensed more, and so obscure 

 emission.'"" 



All these examples seemed to justify the hypothesis 

 that the nature of electrics is such that material 

 effluvia are emitted when electrics are rubbed, and 

 that the effluvia are rarer than air. Gilbert realized 

 that as yet he had not explained electrical attraction, 

 only that the pull can be screened. The puU must be 

 explained by contact forces,'"' as Aristotle '"* and 



«' M: pp. 90, 92, 95. 



<» M: pp. 83, 84, 85. 



^ M: p. 84. 



ii» M: pp. 84, 89. .Sec also .'\iistotlc, np. cil. (footnote 45), 

 Meteorotogica^ bk. 4. 



'<" M: p. 90. 



102 M: pp. 84, 85. 



"« M: p. 84. 



'»< M: p. 90. See also p. 95. 



•»* M: pp. 78, 85-86, 91. (see particularly the heated amber 

 experiment described on p. 86). 



'»6 M: p. 87. 



10' M: p. 92. 



'<« Aristotle, Physics, translated by P. H. VVickstced and F. 

 M. Cornford, Loeb Classical Library, London, 1934, bk. 7, 

 ch. 1, 242b25, 



Aquinas '"" had argued. .Accordingly, he declared, 

 the eflfluvia, or '"spiritus," "° emitted take "hold of 

 the bodies with which they unite, enfold them, as it 

 were, in their arms, and bring them into union with 

 the electrics." '" 



It can be seen how this uniting action is effected 

 if objects floating on water arc considered, for solids 

 can be drawn to solids through the medium of a 

 fluid. "- A wet body touching another wet body 

 not only attracts it, but moves it if the other body is 

 small, "■' while wet bodies on the surface of the water 

 attract other wet bodies. A wet object on the surface 

 of the water seeks union with another wet object 

 when the surface of the water rises between both: at 

 once, "like drops of water, or bubbles on water, they 

 come together.""'' On the other hand, "a dry body 

 does not move toward a wet, nor a wet to a dry, but 

 rather they seem to go away from one another.""' 

 Moreover, a dry body does not move to the dry rim 

 of the vessel while a wet one runs to a wet rim."* 



By means of the properties of such a fluid, Gilbert 

 could explain the unordered coming-together that 

 he called coacervation."' Different bodies have 

 different eflluvia, and so one has coaccrvation of 

 different materials. Thus, in Gilbert's philosophy 

 air was the earth's effluvium and was responsible for 

 the unordered motion of objects towards the earth."* 



The analogy between electric attraction and fluids 

 is a most concrete one, yet lying beneath this image is 

 a hypothesis that is difl^cult to fix into a mechanical 

 system based upon contact forces. This is the assump- 

 tion that under the proper conditions bodies tend 

 to move together in order to participate in a more 



'™ St. Thomcis Aquinas, op. cil. (footnote 19), vol. 2, Physicorum 

 Arislolelis exposilio, lib. 7, lect. 2 (In moventibus ct motis non potest 

 procedi in infinitum, sed oportet devenirc ad aliquid priminn 

 movens immobile), cap. d, p. 96. 



"» M: p. 94. 



1" M: p. 95. 



"2 M: p. 93. 



I" M: pp. 92, 93. 



"* M: p. 93. 



"* M: p. 94. 



>i» M: p. 94. 



i"M: p. 97. 



"9 M: p. 92 (sec also p. 339). Although Gilbert docs not make 

 it explicit, this would solve the medieval problem of gravitation 

 witliout resorting to a Ptolemaic univei-sc. In addition, since 

 coaccrvation is electric, and electric forces can be screened, 

 it should have been possible to reduce tl>c downward motion 

 of a body by screening! 



PAPER 8: NATURAL PHILOSOPHY OF WILLIAM GILBERT 



133 



