LESSONS IN ELECTRICITY. 



301 



my hand closely over its surface, rcmor- 

 iug from that surface nearly the whole of 

 its electricity. I hand you the tube 

 again, and you again excite it. You 

 hand it to me, and I again discharge it. 

 In each case, therefore, you excite an un- 

 clectrified glass tube, and in each case 

 the tube leaves behind upon the rubber 

 an amount of negative electricity equal in 

 quantity to the positive carried away. 

 By thus adding charge to charge, the 

 rubber is rendered highly electrical ; and 

 even should its insulating power bo im- 

 paired by the amalgam, it can now afiord 

 to yield a portion of its electricity to your 

 hand and body, and still powerfully repel 

 rubbed gutta-percha. The principle, 

 which might be further illustrated, is ob- 

 tiously the same as that applied in the 

 case of the comb. 



12. What is Electricity ? 



Thurs far we have proceeded from fact 

 (o fact, acquiring knowledge of a very 

 valuable kind. But facts alone cannot 

 satisfy us. We seek a knowledge of the 

 principles which lie behind the facts, and 

 which aie to be discerned by the mind 

 alone. Thus, having spoken as we have 

 done, of electricity passing hither and 

 thither, and of its being prevented from 

 passing, hardly any thoughtful boy cr 

 giri can avoid asking what is it that thus 

 passes ? what is electricity ? Boyle and 

 Newton betrayed their need of an answer 

 to this question when the one imagined 

 his unctuous threads issuing from and re- 

 turning to the electrified body ; and \\hen 

 the other imagined that an elastic fluid 

 existed which penetrated his rubbed glass. 



When I say " imagined " I do not in- 

 tend to represent the notions cf these 

 great mew as vain fancies. "Without im- 

 agination we can do nothing here. By 

 imagination I mean the power of pictur- 

 ing mentally things which, though they 

 have an existence as real as that of the 

 world around us, cannot be touched 

 directly by the organs of sense. I mean 

 the purified scientific imagination, with- 

 out the exercise of which we cannot take 

 a, single step into the region of causes and 

 principles. 



It was by the exercise of the scientific 

 imagination that Franklin devised the 

 theory of a single electric fluid to explain 

 electrical phenomena. This fluid he sup- 



posed to be self-repulsive, and diffused 

 in definite quantities through all bodies. 

 lie supposed that when a body has more 

 than its proper share it is positively, 

 when less than its proper share it is nega- 

 tively electrified. It was by the exercise 

 of the same faculty that Symmer devised 

 the theory of two electric fluids, each 

 self-repulsive, but both mutually attrac- 

 tive. 



At first sight Franklin's theory seems 

 by far the simpler of the two. But its 

 simplicity is only apparent. For though 

 Franklin assumed only one fluid, he was 

 obliged to assume three distinct actions. 

 Firstly, he had the self -repulsion of the 

 electric particles. Secondly, the mutual 

 atraction of the electric particles and the 

 oonderablc particles of the body through 

 which the ciectnoitv was diffused. 

 Thirdly, these two assumptions when 

 strictly followed out ica^ 10 the unavoid- 

 able conclusion that the material particles 

 also mutually repel each other. Thus the 

 theory is by no means so simple as it ap- 

 pears. 



The theory of Symmer, though at first 

 pight the most complicated, is in reality 

 by far the simpler of the two. Accord- 

 ing to it electrical actions are produced 

 by two fluids, each .self-repulsive, but 

 both mutually attractive. These fluids 

 cling to the atoms of matter, and carry 

 the matter to which they cling along with 

 them. Every body, in its natural condi- 

 tion, possesses both fluids in equal quan- 

 tities. As long {is the fluids are mixed 

 together they neutralize each other, the 

 body in which they arc thus mixed being 

 in its natural or unelectrical condition. 



By friction (and by various other 

 means) these two fluids may be torn 

 asunder, the one clinging by preference to 

 the rubber, the other to the body rubbed. 



According to this theory there must 

 always be attraction between the rubber 

 and the body rubbed, because, as we 

 have proved, they arc oppositely electri- 

 fied. This is in fact the case. And 

 mark what I now say. Over and above 

 the common friction, this electrical at- 

 traction has to be overcoaao v /fene^ r< wa 

 rub glass with silk, cr 88fiiL^g~tfi* .^itli. 

 flannel. 



You arc too young to fully grasp this 

 subject yet ; and indeed it would lead us 

 too far away to enter fully into it. But 



