154 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XIII.) 



of the substances referred to, as, for instance, the metals, water, air, &c., would lead 

 to many very curious theoretical generalizations, which I must not indulge in here. 

 One point, however, I shall venture to refer to. Conduction appears to be essentially 

 an action of contiguous particles, and the considerations just stated, together with 

 others formerly expressed (1326. 1336, &c.), lead to the conclusion, that all bodies 

 conduct, and by the same process, air as well as metals ; the only difference being in 

 the necessary degree of force or tension between the particles which must exist be- 

 fore the act of conduction or transfer from one particle to another can take place. 



1612. The question then arises, what is this limiting condition which separates, 

 as it were, conduction and insulation from each other ? Does it consist in a difference 

 between the two contiguous particles, or the contiguous poles of these particles in 

 the nature and amount of positive and negative force, no communication or discharge 

 occurring unless that difference rises up to a certain degree, variable for different 

 bodies, but always the same for the same body ? Or is it true that, however small the 

 difference between two such particles, if time be allowed, equalization of force will 

 take place, even with the particles of such bodies as air, sulphur or lac? In the first 

 case, insulating power in any particular body would be proportionate to the degree 

 of the assumed necessary difference of force ; in the second, to the time required to 

 equalize equal degrees of difference in different bodies. With regard to airs, one is 

 almost led to expect a permanent difference of force, but in all other bodies, time 

 seems to be quite sufficient to ensure, ultimately, complete conduction. The dif- 

 ference in the modes by which insulation may be sustained, or conduction effected, 

 is not a mere fanciful point, but one of great importance, as being essentially con- 

 nected with the molecular theory of induction, and the manner in which the particles 

 of bodies assume and retain their polarized state. 



^ xi. Relation of a vacuum to electrical phenomena. 



1613. It would seem strange if a theory which refers all the phenomena of insu- 

 lation and conduction, i. e. all electrical phenomena, to the action of contiguous par- 

 ticles, were to omit to notice the assumed possible case of a vacuum. Admitting that 

 a vacuum can be produced, it would be a very curious matter indeed to know what 

 its relation to electrical phenomena would be ; and as shell-lac and metal are directly 

 opposed to each other, whether a vacuum would be opposed to them both, and allow 

 neither of induction or conduction across it. Mr. Morgan*' has said that a vacuum 

 does not conduct. Sir H. Davy concluded from his investigations, that as perfect a 

 vacuum as could be madcj- did conduct, but does not consider the prepared spaces 

 which he used as absolute vacua. In such experiments I think I have observed the 



* Philosophical Transactions, 1785, p. 272, t Ibid. 1822, p. 64. . 



