42 2 THE POPULAR SCIENCE MONTHLY 



two kinds of electrification, wliicli we will distinguish by the terms 

 positive and negative, and we will call any body positively electrified if 

 it is repelled by a glass rod which has been rubbed with silk, and we 

 will call any body negatively electrified if it is repelled by sealing wax 

 which has been rubbed with cat's fur. These are to-day our definitions 

 of positive and negative electrical charges. Notice that in setting 

 them up we propose no theory whatever of electrification, but content 

 ourselves simply with describing the phenomena. In the next place it 

 was surmised by Franklin in 1750, and proved very accurately by 

 Faraday in 1837, that when glass is positively electrified by rubbing 

 it with silk, the silk itself takes up a negative charge of exactly the 

 same amount as the positive charge received by the glass, and, in gen- 

 eral, that positive and negative electrical charges always appear simul- 

 taneously and in exactly equal amounts. So far, still no theory ! But 

 in order to have a rational explanation of the phenomena so far con- 

 sidered, particularly this last one, Franklin now made the assumption 

 that something which he chose to call the electrical fluid or " electrical 

 fire " exists in normal amount as a constituent of all matter in the neu- 

 tral, or uneleetrified state, but that more than the normal amount in any 

 body is manifested as a positive electrical charge, and less than the nor- 

 mal amount as a negative charge, -^pinus, an English admirer of Frank- 

 lin's theory, pointed out that, in order to account for the repulsion of 

 two negatively electrified bodies, it was necessary to assume that matter, 

 when divorced from Franklin's electrical fluid, was self-repellent, i. e., 

 that it possessed properties quite different from those which are found 

 in ordinary uneleetrified matter. In order to leave poor old matter, 

 whose independent existence was thus threatened, endowed with its 

 familiar old properties, other physicists of the day preferred to assume 

 that matter in a neutral state shows no electrical properties because it 

 contains as constituents equal amounts of two fluids which they called 

 positive and negative electricity, respectively, and that a positively 

 charged body is one in which there is more positive than negative, and 

 vice versa. The two theories are not, at bottom, very different, since 

 Franklin's modified one-fluid theory required that matter, when di- 

 vorced entirely from the electrical fluid, have exactly the same proper- 

 ties which the two-fluid theory ascribed to negative electricity, barring 

 only the property of fluidity; so that the most important distinction 

 between the theories was that the two-fluid theory assumed the exist- 

 ence of three distinct entities, named positive electricity, negative 

 electricity and matter, while the one-fluid theory reduced these three 

 entities to two which Franklin called matter and electricity, but which 

 might perhaps as well have been called positive electricity and negative 

 electricity, uneleetrified matter being reduced to a mere combination 

 of these two. Whether the electrical fiuid (or fluids) was supposed to 



