373. =Franklin's Theory.=--In § 372 is developed the theory now commonly received in regard to electricity. The theory of Franklin was different. He supposed that there is but one electric fluid, and that all bodies are in their usual state charged with a certain portion of it, some having more than others, according to their capacity for electricity. While a body is in its usual state there is no manifestation of electricity. The fluid is in a quiescent condition, because its particles are prevented from repelling each other by the attraction which exists between them and the particles of the substance. But this quiescence can be disturbed by friction and other causes. Thus if a glass rod be rubbed with a piece of silk, the natural equilibrium is disturbed, the glass having an excess and the cloth a deficiency of electricity. The glass is therefore said to be _positively_ and the cloth _negatively_ electrified. The equilibrium can be restored in the case of a positively electrified body by having its excess drawn off, and in the case of a negatively electrified body by having its deficiency made up by receiving electricity from other bodies. Though this theory is discarded, the terms positive and negative derived from it are retained, being applied to the two fluids[6] or electricities, and they are often designated by the two signs + and -. 374. =Upon What the Kind of Electricity Excited Depends.=--It depends on what a substance is rubbed with whether vitreous or resinous electricity is excited in it. Thus smooth glass rubbed with woolen cloth or silk will be positively electrified; while if it be rubbed upon the back of a cat it will exhibit negative or resinous electricity. So, also, if a resin, as gumlac or sealing-wax, be rubbed with silk or woolen cloth, it will be charged with resinous electricity, but it will be charged with vitreous or positive if it be rubbed with sulphur. The terms vitreous and resinous are therefore incorrect, for they are based upon the idea that one kind of electricity is always excited on glass, whatever the friction may be made with, and that the other kind is always excited on resins. The most decided illustration of the incorrectness of these terms we have in the fact, that while smooth glass rubbed with silk or woolen cloth becomes charged with positive (vitreous) electricity, roughened glass rubbed with the same gives us negative (resinous) electricity. Below I give a table of substances, any one of which has positive electricity developed on it when it is rubbed with any substance below it on the list, and negative when rubbed with any substance above it: 1. Cat-skin. 2. Polished glass. 3. Woolen cloth. 4. Feathers. 5. Wood. 6. Paper. 7. Silk. 8. Sealing-wax. 9. Amber. 10. Roughened glass. 11. Sulphur.