Silks of all kinds; the hair and fur of animals, paper, sulphur, and some other minerals; most of the precious stones; the paste of which false gems are made; and many other substances used by us in the common affairs of life, are susceptible of electrical excitement; among domestic animals the cat furnishes a remarkable instance. When dry and warm, the back of almost any full-grown cat (the darker its color the better) can be excited by rubbing it with the hand in the direction of the hair, a process which is accompanied with a slight snapping noise, and in the dark by flashes of pale blue light. When a piece of glass is rubbed with silk, or a stick of red sealing-wax with woollen cloth, each substance acquires the property of attracting and repelling feathers, straws, threads of cotton, and other light substances; the substances just mentioned as highly electric are, however, merely specimens. All objects, without exception, most probably are capable of being electrically excited; but some require more complicated contrivances to produce it than others. _Electric_, having the properties of electricity. _Susceptible_, disposed to admit easily. _Repelling_, the act of driving back. _Complicated_, formed by the union of several parts in one. Is there not a machine by which we are enabled to obtain large supplies of electric power at pleasure? Yes; the electrical machine. It is made of different forms and sizes: for common purposes those of the simplest form are the best. A common form of the machine consists of a circular plate of glass, which can be turned about a horizontal axis by means of a suitable handle. This plate turns between two supports, and near its upper and lower edges are two pairs of cushions, usually made of leather, stuffed with horse-hair and coated with a mixture of zinc, tin, and mercury, called an _amalgam_. These cushions are the rubbers for producing friction, and are connected with the earth by means of a metal chain or rod. Two large hollow cylinders of brass with globular ends, each supported by two glass pillars, constitute the reservoir for receiving the electricity. They are called the _prime conductors_, and are supplied with U-shaped rods of metal, furnished with points along their sides, called _combs_, for the purpose of receiving the electricity from the glass plate, the arms of the U being held upon either side. The other ends of the conductors are connected by a rod from the middle of which projects another rod terminating in a knob, for delivering the spark. On turning the plate, a faint snapping sound is heard, and when the room is darkened, a spark is seen to be thrown out from the knob projecting from the _prime conductors_.