IN whatever manner, and by whatever bodies, elec tricity is produced, its general effects are always the same. Electric attraction, and electric light, arc both produced, whether we make the experiment with excit ed glass, or excited amber. The electricities, however, generated from these two substances, have different cha racters, which manifest themselves by an attentive exa mination of their effects. If two small balls, turned from the pith of elder, are suspended by silver wires, or fine threads, as is represented in Plate CCXLIII. Fig. 1. they will remain in contact. But as soon as a tube of glass, excited by friction, is presented to them, the balls diverge, and separate from each other, as in the figure. The same divergence is produced, by presenting excited amber or wax to the pith balls. But if, when the pith balls are in a state of divergence by the ap plication of excited glass, we present also to them a stick of excited wax, the electricity will disappear, and the balls will collapse. If, on the contrary, when the divergency of the balls has been produced by the appli cation of excited wax, we present to them a tube of excit ed glass, the balls will collapse as before. If the excir ed wax, and the excited glass tube, are presented to gether to the pith balls, no divergency will take place. Here then we have an unequivocal indication of two different kinds of electricity, viz. that of the 'glass and of the sealing-wax, the one always counteracting the effect of the other. These two kinds of electricity were denominated the -vitreous and the resinous electricities, from the bodies by which they were produced ; but as it has been found, that resinous electricity can be obtained by the excitation of glass, and vitreous electri city from the excitation of resinous bodies, the terms of vitreous and resinous can no longer be admitted. The terms positive and negative, or plus and minus, have therefore been universally employed to denote the vitre ous and resinous electricities.
The substitution of these new terms will appear still more appropriate, when we consider that, in every case, where a body is excited, both positive and negative electricity are simultaneously produced, and in almost every case can be rendered visible by the application of the pith balls. This truth admits of a very simple proof, by exciting sealing-wax with dry flannel, and applying them together and separately to the pith balls. When they are applied separately, the flannel will indicate a positive electricity, like a tube of glass excited with silk, while the wax will indicate negative electricity. When they are presented together to the pith balls, no effect whatever will take place, so that the positive electricity of the flannel is in equilibrium with, or equal in effect to, the negative electricity of the wax. In the same manner, when the black ribbon is separated from the white ribbon, after being, excited in the Mantic already described, the black ribbon will be found negatively, and the white ribbon positively eleettified.
But the positive and negative electricities are not only 'simultaneous phenomena ;—any electric can be made to either or the eleet•ieities, according to the nature of the substance with which it is excited, or the state of the surface of the electric itself. This im portant discovery we owe to :11r Canton, who established it by numerous experiments. Polished glass, for ex ample, is positively electrified when rubbed with flannel, but negatively electrified when rubbed with the back of a cat ; while rough glass acquires positit e electricity by the friction of dry oiled silk, and negative electricity by the friction of flannel.
The results of the numerous experiments which have been made on this subject, are contained in the fol lowing Table, altered and greatly enlarged from the Ta ble drawn up by the late Mr Cavallo, front the expel i ntents of various electricians.
In the repetition of some of the preceding experiments, the minutest attention must be paid to the state of the bodies which are rubbed together, and to the manner in which the friction is applied. When a stick of sealing wax is rubbed with an iron chain, the electricity of the wax will be positive when its own surface is scratched, and negative when it is smooth. When a silk ribbon is drawn lengthwise over a part of another ribbon of the same length, the ribbon which has been rubbed along its whole length becomes positively and the other nega tively electrified. In like manner, when the whole length of the bow of a violin is drawn over a limited part of the string, the hairs of the bow exhibit a posi tive, and the string a negative electricity, the body whose excited portion of the least extent being generally ne gatively electrified. Some other examples of a similar kind have been given in our historical account of the ex periments of Cigna.
In exciting metals with woollen cloth, similar anoma lies have been observed by Hauy; the same piece of metal, even when placed in similar circumstances, some times acquiring positive and sometimes negative elec tricity. M. He Luc has observed a similar irregularity in the friction of glass and caoutchouc. This anomaly, however, is seen in a most beautiful manner in the mi neral commonly called cyanite, which was first described by the younger Saussure, under the name of sappare. Some of the crystals of this mineral acquire positive electricity by friction, while others acquire negative electricity; and in some of them the one species of elec tricity is found on one face, and the opposite species on the other face, although neither the eye nor the touch could discover any difference either in the lustre or the polish of the two faces. From this property Hauy has called the mineral disthene, which signifies having two The effect of minute circumstances in the simultane ous production of positive and negative electricity, is excellently illustrated by the experiments of Mr Ca vallo, on the electricity of glass tubes containing mer cury, and hermetically sealed. These tubes were about 31 inches long, less than half an inch in diameter, and half an inch thick. Having put about three-fourths of an ounce of mercury into the tube, he boiled it, and, when the air was rarefied, the tube was hermetically sealed. After cleaning and warming the tube, he brought it into a horizontal position, and excited it by the friction of the mercury, which took place in elevating and de pressing either end alternately. When the tube after this kind of excitation was brought into a vertical posi tion, the end containing the mercury was electrified po sitively, and all the rest of the tube negatively. When the positive end was raised, the mercury flowed to the negative end, but the end where the mercury now stood acquired a positive electricity, and the rest of the tube a negative one. When the positive end where the mer gury first stood, was elevated without touching it with the hand, it exhibited only a very slight degree of nega tive electricity ; and if the mercury was made to flow back to it, and again retire from it, its electricity was positive, while by elevating it with the finger it always became strongly negative.