ELECTRICITY. 133 



coating of the two boards, the equilibrium was re-established, and he received 

 the shock produced by the passage of the electric fluid from the one to the other. 



Many curious experiments were exhibited with this apparatus. They found 

 that the two boards, when electrified, strongly attracted each other, and would 

 have rushed together if they had not been prevented by the strings. Some- 

 times, when the charge was strong, the intervening plate of air was not suf- 

 ficiently impermeable to resist the mutual attraction of the opposite electricities, 

 and a spontaneous discharge would take place through it. They considered 

 these two plates to represent the state of the clouds and the earth during a 

 thunder-storm ; the clouds being always charged with one kind of electricity, 

 and the earth with the other, while the body of atmosphere between them was 

 analogous to the stratum of air between the two boards. When the charges 

 of the earth and clouds become so strong that the air can no longer resist the 

 passage of the electric fluid through it, a spontaneous discharge ensues, the 

 fluid is seen in its passage by the light it evolves, and the violent displacement 

 of the air produced in its passage causes the thunder. 



From these experiments, ./Epinus inferred that the phenomena of the Leyden 

 jar was not owing, as Franklin supposed, to any peculiar attraction of the 

 glass for the electric fluid ; for, since a plate of air might be charged as well 

 as a plate of glass, that property must be common to them, and was not pecu- 

 liar to the glass. He inferred, therefore, that this impermeability was a prop- 

 erty of all non-conductors ; and, since they can all receive electricity to a cer- 

 tain degree, it must consist in the difficulty and slowness with which the elec- 

 tric fluid moves in their pores, whereas, in perfect conductors, it meets with 

 no obstruction at all.* 



./Epinus brought to the investigation of the Franklinian theory of electricity 

 those mathematical attainments in which its illustrious founder was deficient. 

 The manner in which that theory had been assailed by its opponents, and de- 

 fended by its partisans, was such as might have allowed interminable contro- 

 versy. ./Epinus first reduced its principles to exact mathematical statement, 

 with a view to ascertain whether the consequences deducible from them, by 

 rigorous calculation, should be in accordance with the observed phenomena, 

 not only in their general character, but in their numerical quantity. He as- 

 sumed, according to Franklin's hypothesis, that the molecules of the electric 

 fluid were self-repulsive, and that they were attracted by those of the bodies 

 on which they were diffused. He found, however, that the phenomena could 

 not be explained on these suppositions, unless it were also assumed that be- 

 tween the matter composing the masses of different bodies there existed a mu- 

 tually repulsive force, acting at sensible distances. At first he recoiled from an 

 assumption in direct opposition to the known properties of matter ; but the ne- 

 cessity of its admission, in order to give consistency and validity to the Frank- 

 linian theory, appears at length to have reconciled him to it. 



Ths investigation of the physical relation between the principle of heat and 

 that of electricity, had attracted the attention of experimental philosophers at a 

 very early period in the history of electrical research. Beccaria suspected 

 that heat might itself be an immediate means for the development of electricity, 

 and made some experiments to illustrate this. He soon, however, relinquished 

 the inquiry, concluding that, in cases where the appearance of electricity fol- 

 lowed the application of heat, the effect was due to evaporation, or other 

 physical agents, which ensued. Priestley observed that heat had some relation 

 to the conducting power of bodies, since, by the elevation of temperature, that 

 quality was improved. 



* jEpini Tentamcn, &c. Petersburg, 1759, p. 82, 83. 



