375. =Conductors and Non-Conductors.=--Electricity passes over the surface of some substances very readily; while over others it moves with very great difficulty, and therefore very slowly and sparingly. The former are termed conductors, and the latter non-conductors. As in the case of heat, so with electricity there are no substances which are wholly non-conducting. The best of all the conductors are the metals, those least liable to oxydation being the most perfect. Next come charcoal, water, living substances, flame, smoke, steam. The best non-conductors are gumlac and gutta-percha. Then come amber, resins, sulphur, glass, silk, wool, hair, feathers, cotton, paper. Non-conductors are sometimes called _insulators_, from the Latin word _insula_, as they serve to confine electricity within certain bounds, and prevent its escaping. Thus in the experiments with pith balls, already cited, the silk threads by which they are suspended prevent the electricity from escaping from them. So the glass knobs on which the wires of the telegraph rest are insulators, preventing the electric fluid from escaping down the poles into the ground. [Illustration: Fig. 258.] 376. =Electricity Always on the Surface.=--There is a marked difference between heat and electricity in the manner in which they are disposed of. Heat pervades all the particles of substances, and in its conduction spreads through them, while electricity in its ordinary movements operates altogether on the surface. A hollow ball, therefore, can contain as much electricity as a solid, and a hollow conductor of electricity is just as effectual as a solid one. The following experiment exhibits in a very striking manner this disposition of electricity to occupy the surface alone: Let _a_, Fig. 258, be a metallic ball supported by a glass stand, _b_; and let _c c_ be metallic caps which will just cover the ball, having non-conducting handles, either glass or gumlac. Now, after having charged the ball with electricity, let the caps held by the insulating handles be carefully placed over the ball. On withdrawing them it will be found that the electricity of the ball has all passed to the outer surface of these caps.