Although we discover, in all the processes of nature, the manifestations of this principle or force in its characteristic conditions, it will be necessary, before we regard the great phenomena, to examine the known sources from which we can most readily evoke the mighty power of electricity. If we rub a piece of glass or resin, we readily render this agent active; these substances appear, by this excitement, to become surrounded by an attractive or a repellent atmosphere. Let us rub a strip of writing paper with Indian rubber, or a strip of Gutta Percha with the fingers, in the dark, and we have the manifestation of several curious phenomena. We have a peculiar attracting power; we have a luminous discharge in the shape of a spark; and we have very sensible evidence of muscular disturbance produced by applying the knuckle to the surface of the material. In each case we have the development of the same power. Every substance in nature is an electric, and, if so disposed that its electricity may not fly off as it is developed, we may, by friction, manifest its presence, and, indeed, measure its quantity or its force. All bodies are not, however, equally good electrics; shell-lac, amber, resins, sulphur, and glass, exhibiting more powerfully the phenomena of frictional or mechanical electricity, than the metals, charcoal, or plumbago. Solid bodies allow this peculiar principle to pass along them also in very different degrees. Thus electricity travels readily through copper and most other metals, platinum being the worst metallic conductor. It also passes through living animals and vegetables, smoke, vapour, rarified air, and moist earth; but it is obstructed by resins and glass, paper when dry, oils, and dry metallic oxides, and in a very powerful manner by Gutta Percha.[137] If, therefore, we place an electric upon any of those non-conducting bodies, the air around being well dried, we are enabled to gather a large quantity of the force for the production of any particular effect. Taking advantage of this fact, arrangements are made for the accumulation and liberation at pleasure of any amount of electricity.