3 6 THE POPULAR SCIENCE MONTHLY. 



of these great men as vain fancies. Without imagination we can do 

 nothing here. By imagination I mean the power of picturing men- 

 tally things which have an existence as real as that of the world 

 around us, but which cannot be touched directly by the gross bodily 

 organs of sense. I mean the purified scientific imagination, without 

 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 phe- 

 nomena. This fluid he supposed to be self-repulsive, and diffused in 

 definite quantities through all bodies. He supposed that when a 

 body has more than its proper share it is positively, when less than 

 its proper share it is negatively, 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 attractive. 



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. Two 

 of these were the mutual repulsion of the electric particles among 

 themselves, and the mutual attraction of the electric particles and the 

 ponderable particles of the body through which the electricity is dif- 

 fused. These two assumptions, moreover, when strictly followed out, 

 lead to the unavoidable conclusion that the material particles must 

 also mutually repel each other. Thus the theory is by no means so 

 simple as it appears. 



The theory of Symmer, though at first sight the most complicated, 

 is in reality by far the simpler of the two. According to it electrical 

 actions are produced by two fluids, each self-repulsive, but both mu- 

 tually 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 condition, possesses both fluids in equal quantities. As 

 long as the fluids are mixed together they neutralize each other, the 

 body in which they are 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 

 are oppositely electrified. 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 overcome whenever we rub glass with silk, or seal- 

 ing-wax with flannel, 



You are too young to fully grasp this subject yet ; and indeed it 

 would lead us too far away to enter fully into it. But I will throw 

 out for future reflection the remark that the overcoming of the ordi- 



