462 ELECTRICAL HYPOTHESES. [226. show signs of electric excitement on being separated (257). The friction of glass against metal spread over silk is attended by a more powerful development of electricity than when silk alone is used; and an amalgam consisting of 1 part of tin, 2 of zinc, and 6 of mercury, rubbed to fine powder and mixed with a little lard, is found to be highly effectual in exalting the quantity which is developed. The same substance, however, does not always manifest the same electrical condition when rubbed; glass when rubbed upon silk becomes vitreously excited; but if rubbed on the fur of a cat it exhibits resinous electricity. Metallic bodies, when their surface is perfectly free from oxide, become negatively electric when rubbed by imperfect conductors such as silk, fur, wood, cork, or ivory. The amount of friction necessary to produce electric excitement is exceedingly small; the mere drawing of a handkerchief across the top of the electroscope (fig. 170), or even across the clothes of a person insulated by standing on a cake of resin, or on a stool with glass legs, provided he touch the cap of the instrument, is sufficient to cause divergence of the leaves. The simple act of drawing off silk stockings, or a flannel waistcoat, or the combing of the hair in frosty weather, frequently occasions the snapping and crackling noise due to the electric spark; and the stroking of the fur of a cat at such a season is well known to produce similar effects. (227) Electrical Hypotheses.-These various phenomena have been accounted for by two principal hypotheses. One of these, commonly known as the theory of one fluid,' is due to Franklin. Electricity, upon this view, is supposed to be a subtle imponderable fluid, of which all bodies possess a definite share in their natural or unexcited state. By friction or otherwise, this normal state is disturbed. If the body rubbed receive more than its due share, it acquires vitreous electricity, or, in the terms of Franklin, becomes electrified positively, or + ; whilst at the same time the quantity of electricity in the rubber which becomes resinously charged is supposed to be diminished, and thus the rubber acquires a negative or state. Franklin sup posed the particles of the electric fluid to be highly self-repulsive, and to be powerfully attractive of the particles of matter. The other hypothesis, the theory of two fluids,' was originally proposed by Dufay. According to this view there are two electric fluids, the vitreous and the resinous, equal in amount but opposite in tendency; when associated together in equal quantity they neutralize each other perfectly; a portion of this compound fluid pervades all substances in their unexcited state. By friction the 227.] DUAL CHARACTER OF ELECTRICITY. 463 compound fluid is decomposed; the rubber acquires an excess of one fluid, say the resinous, and thus becomes resinously excited; the body rubbed takes up the corresponding excess of vitreous electricity, and becomes excited vitreously to an equal extent. Upon this view the particles of each fluid are self-repulsive, but powerfully attract those of the opposite kind. The language of either theory may be employed in order to distinguish the two kinds of electricity: the term vitreous or positive may be used indifferently for one kind, and resinous or negative for the other kind, provided it be borne in mind that positive and negative are mere distinguishing terms: negative electricity being as real a manifestation as the positive. It is manifest that one or other of these hypotheses must be false, yet either will serve to connect the facts together. The supposition of an electric fluid is, notwithstanding, gradually being abandoned. The supposition of a gravitative fluid might, with nearly as much propriety, be insisted on to explain the phenomena of gravitation, or a cohesive fluid to account for those of cohesion. FIG. 173. Electricity is now regarded as a stress in a medium, remarkable for the peculiar form of action and reaction which it exhibits. This kind of action and reaction follows the same law of equality and opposition in its manifestations as that which is exhibited more obviously in the phenomena of mechanics. Whenever vitreous electricity is manifested at one point, a corresponding amount of resinous electricity is invariably developed in its vicinity, reacting against it, and thus enabling its presence to be recognised, although this reaction may not be immediately perceptible. The phenomena of vitreous and resinous electricity may be rudely but not inaptly illustrated by those of elasticity exhibited by an ordinary spring, as shown at s, fig. 173. The spring in its unstretched state may represent the body in its unelectrified condition; it then displays nothing of the peculiar power that it possesses. The spring cannot be stretched from one extremity only; but if fixed at one end, as by hooking it to the pin, P, a weight, w, may be applied to the other end, and it will seem to be stretched by one weight only. In reality, however, it is not W so; for by substituting at v a weight equal in amount to that at w, instead of the fixed point, P, the tension of the spring remains 464 ELECTRICAL INDUCTION. [227. unaltered, but a reaction, equal and opposite to the original action of the weight, w, is instantly rendered evident. So it is with electricity; cases not unfrequently occur where one kind only of electricity seems to be present, but a careful examination will always detect an equal amount of the opposite kind. This essential character of action and reaction in the electrical force will be more clearly manifested in the following remarks and experiments. (228) Electrical Induction.—In the preceding cases the elec tricity has been excited by friction and communicated to other bodies by contact. An insulated charged body, however, exerts a remarkable action upon other bodies in its neighbourhood. Long before contact occurs, the mere approach of an excited glass tube towards the electroscope causes divergence of the leaves, and on removing the glass tube, if it have not been allowed to touch the cap of the instrument, all signs of disturbance cease. The following mode of performing the experiment will afford a meaus of examining this action of an electrified substance upon objects at a distance : FIG. 174. + + Place two cylinders of wood, or of metal, each supported on a varnished stem of glass, so as to touch each other end to end (fig. 174, 1); from the outer extremity of each suspend a couple of pith balls by a cotton thread, and bring the excited glass tube near one end of the arrangement as shown Electric disturbance will be shown by the repulsion of both pairs of balls. Separate the two cylinders without touching the conducting portion, and then remove the glass tube; the balls will still continue to diverge (3). But let 2. + 3. ++ + at 2. the glass be again brought near; the balls on the cylinder originally nearest the glass will collapse, showing this cylinder to be negatively excited, while the same excited glass will cause the balls on the further cylinder to diverge from the presence of positive electricity. Again, remove the glass altogether, and bring the two cylinders into contact; a spark may generally be seen to pass between them, and both pairs of balls will immediately collapse and continue at rest. The entire amount of electricity existing upon the two cylinders taken together remains the same throughout the whole period of the experiment, but its distribution 228.] CHARGE OF GOLD-LEAF ELECTROSCOPE. 465 is altered, as is shown by the position of the signs + and -. The experiment may be explained in the following manner :-Suppose the two cylinders to be in the neutral state (No. 1); on bringing the excited glass tube near to them, a portion of the negative electricity appears to be drawn towards the end of the cylinder nearest to the glass, as in No. 2, whilst the corresponding quantity of disengaged positive electricity causes the balls on both cylinders to diverge the moment the glass is removed, the negative electricity redistributes itself as in No. 1, and the balls collapse; but if the two cylinders be separated before the glass is removed, and if the excited glass be then withdrawn,* the results will be such as are represented in No. 3, in which the negative electricity on one of the cylinders is more than sufficient to neutralize the positive, and hence the balls diverge negatively; while on the other it is less than sufficient for the positive, consequently the balls diverge with positive electricity. On causing the two cylinders to approach each other when in this state, the two electricities will neutralize each other, and if of sufficient power, the reunion will be attended with a slight spark. This action at a distance of one electrified body upon others in its neighbourhood is termed electrical induction. It is a principle of very extensive application, and indeed it furnishes a key to the explanation of the greater number of electrical phenomena. An instance of electrical induction is afforded in the action of the gold-leaf electroscope. Let 1 (fig. 175) represent the instru FIG. 175. 3 ment in a neutral state. As soon as an excited glass tube, G, is caused to approach the cap of the electroscope, the leaves will * If the glass tube be withdrawn gradually to a certain distance, the balls upon the cylinder nearest the tube will gradually collapse, in proportion as the inductive power is weakened by distance; a portion of the negative electricity being liberated in quantity sufficient to neutralize the free positive charge, and, on completely withdrawing the excited tube, the excess of negative electricity is set free, and the balls now diverge negatively. 466 ELECTRICAL INDUCTION. [228. diverge as at 2. Whilst the glass tube is still near the instrument, let the cap of the electroscope be touched with the hand, so as to uninsulate it for a moment, as at 3, by placing it in communication with the earth through the body, which acts the part of a conductor; the leaves will collapse, and the instrument will seem to be quiescent; now remove the finger from the cap, and then take away the glass tube, G; instantly the leaves diverge, and the electroscope is permanently charged, in consequence of a change in the distribution of the electricity, as represented at 4. Its charge, however, is not positive like that of the glass, but negative; for if the glass be again brought near, the leaves will collapse, while a stick of excited wax will make them open out further. These effects arise from electrical induction. and the process which takes place is believed to be the following. The approach of the tube in the first instance causes the negative electricity to accumulate in the cap, as at 2, where it is retained by a species of attraction, in which condition it is said to be disguised. The leaves, therefore, diverge with a corresponding quantity of positive electricity thus set free; things being in this state, a touch is sufficient to neutralize the excess of positive electricity, as seen in 3, and the instrument appears to be quiescent. Remove first the finger, and then the glass tube, however, and the negative electricity, that had been accumulated on the surface of the cap, spreads over the whole instrument (though in the diagram this is only represented as taking place upon the leaves), and the leaves diverge with negative electricity, as shown at 4. In all these cases, the excited body itself neither loses nor gains electricity by the process just described. The mode in which this transfer of force from a distance is effected still remains to be considered. (229) Faraday's Theory of Induction.—We owe to Faraday & theory of these effects, which has been thus concisely summed up by Snow Harris (Rudimentary Electricity, second ed., pp. 44, 45, 47). Faraday' conceives electrical induction to depend on a peculiar form of physical action propagated between contiguous molecules. In these intermediate molecules, a separation of the opposite electricities takes place, and they become disposed in au alternate series or succession of positive or negative points or poles; this he terms a polarization of the particles, and in this way the force is transferred to a distance. Thus, if in fig. 176, P represent a positively charged body, and a, b, c, d, interme |