IN the few simple experiments of which we have given an account, the reader must already have seen, that electricity is capable of being communicated to other bodies. The pith balls obviously diverge, in con sequence of the electricity which they receive from the excited glass or rosin, and retain it for a considerable time. If we touch any of the pith balls, when in a state of divergence, with a piece of glass free from moisture, or with a dry stick of sealing-wax or sulphur, the diver gency will still continue. The electricity, therefore, possessed by the balls, is not capable of being carried off by passing through these bodies. Glass, sealing-wax, and sulphur, are thence called non-conductors of elec tricity, as they do not possess the power of transmitting the electric matter. If, on the contrary, we touch any of the pith balls with a piece of any metal, with the finger, or with any body containing moisture, the pith balls will instantly lose their electricity, and collapse. The electricity, consequently, which occasioned their divergence, has escaped, or been carried off by those bodies, and hence they have received the name of con ductors. Between these two classes of substances, there are others which do transmit the electric fluid, but with less facility than those which are called conductors, and therefore they have been distinguished by the name of imperfect conductors. Substances called non-conductors, have also received the name of electrics, and sometimes insulators, from their having the property of insulating bodies, or forming a barrier, which prevents the com munication of the electricity of the insulated body to those which surround it. The following Table contains all the substances which have the property of conducting electricity, arranged in the order of their conducting power. We have followed the order assigned to them by Mr Singer.
To these we may add, powdered glass and powdered sulphur, which have been found to be conductors by the experiments of Van Swinden.
Several of the bodies in the preceding table lose their power of conducting electricity when they are made very dry, and therefore they owe their conducting power solely to the moisture, or the fluids which they contain: hence vegetables and living animals, for example, con duct electricity solely in virtue of the juices and fluids which they contain. The conducting power varies also with the temperature. Hot charcoal and hot water trans mit electricity with more facility than when they are cold ; and glass, which is a perfect non-conductor when cold and dry, becomes a tolerably good conductor when heated to redness; and the same change takes place upon resinous bodies when melted, and upon baked wood when heated. Air, however, does not conduct electricity, whatever be its temperature.
Although flame is here enumerated in the list of con ductors, yet it has been shewn by M. Erman, that the insulated flames of wax, oil, alcohol, and hydrogen gas, only conduct positive electricity ; while other substan ces, such as phosphorus, conduct only negative electri city. Mr Cuthbertson had observed, (Practical Electri
city, p. 48,) that when the flame of a common candle was placed midway between two equal balls, one posi tively and the other negatively electrified, the flame was attracted to the negative ball, which became very hot, while the positive ball remained cold.
Alr Braude* has endeavoured to explain these phe nomena in another manner. As some chemical bodies are naturally negative, and others positive, he supposes that the positive will be attracted by the negative ball, and the negative by the positive ball; and in order to ascertain the probability of this conjecture, he placed the flames of various bodies between two insulated brass balls, one of which was electrified positively, and the other negatively. By this apparatus he obtained the following very interesting results : Dr Priestley found the conducting power of charcoal to vary very much in different pieces of that substance; and it has since been found, that this variation arises from, and is proportional to, the different degrees of heat that have been employed in making it.
The conducting property of ice was first discovered by M. Jallabel% and confirmed by Dr Priestley. Al. Achard of Berlin, however, observed, in January 1776, that ice, at the temperature of 15° of Fahrenheit, lost its conducting power, and became an electric. Above this temperature, the ice begins to conduct. This fa culty increases with the temperature, and becomes very great near the boiling point. Al Achard employed, in his experiments, ice perfectly transparent, and free from air-bubbles. To obtain this, he poured distilled IN ,,ter into a vessel, and placed it, in frosty weather, on the win dow of a room tolerably warm, when compared with the external air. Hence the water froze on one side when it was fluid on the other ; and the air, as it was extricat ed, passed into the fluid part, and thus left the ice per fectly transparent, and free from air-bubbles. Ile then formed the ice into a spheroid, and, by rubbing it when in motion about its axis, he was able to produce electri city in great quantities. The following Table contains a list of bodies that are non-conductors, arranged in the order of the resistance which they oppose to the passage of the electric fluid.
employed to denote the space round any electrical body through which its electrical influence extends. Mr Can ton was the first electrician who examined this curious subject, and lie found that all bodies immerged in the electric atmosphere of a positively electrified body, were negatively electrified ; while those which were placed in the atmosphere of a negatively electrified body acquired positive electricity. This may be proved by the very simple experiment of holding a pair of pith balls at some distance from an excited glass tube, and they will be found to diverge with negative electricity. In like manner, they will diverge with positive electricity when placed within the sphere of action of a stick of excited sealing-wax negatively electrified.