_THEORY of Electricity._--The theory most readily understood, and which most satisfactorily explains the various electrical phenomena, is as follows:-- "That every substance and every atom of the world is pervaded by a peculiar, subtle, imponderable fluid which is termed _Electricity_, but which is not known to exist, or remains in a state of _electrical equilibrium_, until evoked by certain causes." The effect of causing a disturbance of this equilibrium is to increase the normal, or natural, electricity in some particles, and to equally decrease it in other particles, i.e. what one loses the other gains. An excess of natural electricity is denoted by the term _positive_, or mathematical symbol (+) while a deficiency is denoted by the term _negative_, or symbol (-). _Like electricities repel each other._ That is to say, two bodies charged with an excess of, or positive, electricity, being brought together repel each other, neither wishing to increase the excess that has been evoked in them. Similarly in the case of two bodies charged with a deficiency of, or negative, electricity, neither wish to add to the deficiency already there. In both these cases there can be no tendency to electrical equilibrium, which is the principle at work. In the former case, there being already too much, more will but increase the disturbance. In the latter case, further deficiency will but add to the irregularity. _Unlike electricities attract each other._ That is to say, if two bodies, one charged with positive, or having an excess of electricity, the other charged with negative, or having a deficiency of electricity, be brought together, they will attract each other; both being desirous of altering their existing state, the one by decreasing its excess, and the other by decreasing its deficiency of electricity. In this case, there will be a tendency to equilibrium, caused by attraction. The earth is supposed to be a vast reservoir of electricity, from which a quantity can be drawn to fill up a deficiency, and which is always ready to receive an excess from other bodies. Every body in nature has its own natural quantity of electricity, and when an object is negatively electrified, or has a deficiency in its normal quantity, there is a tendency to receive a supply from any convenient source. Such an object would receive electricity from the earth if means were afforded; and a body _positively_ electrified, would tend to part with its excess in the same manner. Where such facilities for establishing electrical equilibrium are afforded, the result is the passage of a _current_ of electricity.