Page:Popular Science Monthly Volume 80.djvu/426

two kinds of electrification, which 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 general, that positive and negative electrical charges always appear simultaneously and in exactly equal amounts. So far, still no theory! But in order to have a rational explanation of the phenomena so far considered, 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 neutral, or unelectrified state, but that more than the normal amount in any body is manifested as a positive electrical charge, and less than the normal amount as a negative charge, Æpinus, an English admirer of Franklin'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 unelectrified 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 divorced entirely from the electrical fluid, have exactly the same properties 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 existence 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, unelectrified matter being reduced to a mere combination of these two. Whether the electrical fluid (or fluids) was supposed to