222 PROGRESS OF SCIENCE. 



positively electrified bodies those which had an excess of 

 electric fluid, and negatively electrified bodies those which 

 had less the current being determined by the want of 

 equilibrium (if we may use the term) between them : abun- 

 dance flowing to scarcity and establishing a level. By the 

 SYMMER THEORY, however, the purely conventional terms 

 positive and negative have received a more definite significa- 

 tion : positive electricity or pole being synonymous with 

 vitreous, and negative electricity or pole being synonymous 

 with resinous. For the reader's guidance it is necessary to 

 remind him that, whilst we make use of these terms, we are 

 nevertheless in complete ignorance of what electricity is in 

 itself. We know at present that motion, light, heat, mechanical 

 energy, electricity are interchangeable forces, that is, they can 

 be transformed into one another ; we also know now that 

 electricity can be produced by friction, mechanical energy, 

 magnetism, chemical energy, heat, and that electricity 

 therefore is, like heat, a mode of motion ; but our 

 knowledge goes no further. We really do not know 

 whether its existence depends on one or on two fluids, 

 or whether it is only an affection of matter, or again, 

 whether it is a form of atomic motion (Helmholtz says it is 

 probably as atomic as matter), or a visible manifestation of 

 ethereal energy : all we know is that it is a force presenting 

 itself to us in two distinct modes in other words, we know of 

 two distinct modes of the force. Hence our terms " positive " 

 and "negative." If the positive force be exhibited, the 

 equivalent negative force is called into action, and so the 

 force is essentially polar, having two opposite currents. The 

 exhibition of the force, though always subjected to certain 

 laws, is infinitely varied, and therefore not limited to the 

 uniformly typical phenomena already indicated. For instance, 

 when two bodies are rubbed together, as wax and flannel or 

 glass and silk, both bodies are found electrified; but the rubber 

 will be positively electrified if the rubbed body is negatively 

 electrified, and conversely. Again, upon the nature of 

 the rubber the kind of electricity depends : glass rubbed 

 with silk will be positively electrified (and the silk 

 negatively), rubbed with cat's skin glass will be negatively 



