───────────────────────┼───────────────────────┼─────────────────────── Baked wood │Positive │Silk. „ │Negative │Flannel.” From the above table it appears, that the powers of electric substances vary prodigiously from one another; and that, according to the different rubbers made use of, we may sometimes produce one phenomenon and sometimes another. Hence we have a foundation for classing electric substances according to the various powers they occasionally exhibit; which may be done in the following manner. First. Those which exhibit a _strong_ and _permanent_ attractive and repulsive power, of which the most remarkable is silk. Second. Those which exhibit the electric light, attraction, repulsion, and all the other phenomena of electricity in a very _vigorous_, though _not durable_ manner; of these glass is eminently preferable to all others. Third. Those which exhibit electric appearances _for any length of time_, and which communicate to conducting bodies, _the greatest electric power_.—Of these, the substances called _negative electrics_, such as sealing-wax, resinous substances, and resinous compounds, are the best. Fourth. Those which readily exhibit electrical phenomena by _heating_ and _cooling_.—Of these, the tourmaline[9] is the principal. The best method of disturbing the electric fluid, that is of making it pass from one body to another, is friction. This may be done either by rubbing one electric with another, or with a conductor; but the electricity is generally stronger in the latter case. Other methods for causing electrics to shew electric appearances, are, melting, or pouring a melted electric on another substance, heating and cooling, evaporating or effervescing. CHAP. III. _Of Electrics and Conductors._ All bodies in nature are, with reference to this subject, divided into two classes, _electrics_ and _conductors_. It has been fully demonstrated by experiment, that no substance which is a conductor can be excited so as to exhibit electrical phenomena: and in the same manner it has been found, that no substance which can be excited, is a conductor. But as we have already hinted, there is, strictly speaking, no substance which is a _perfect_ conductor or non-conductor; because, on the one hand, the electric fluid meets with some resistance in its passage through the best conductors; and on the other, it is in part transmitted through, or passes over the surface of, most if not all electrics. The two following lists contain as complete an enumeration of electrics and conductors as the present state of knowledge, in regard to electricity, permits us to make.