‘May not the dark bands be the nodes of undulations arising from similar impulses proceeding from positive and negative discharges? or can the luminous stratifications which we obtain in a close circuit of the secondary coil of an induction apparatus, and in the circuit of a voltaic battery, be the representations of pulsations which pass along the wire of the former, and through the battery of the latter, impulses probably generated by the action of the discharge along the wires?’ The action of magnetism and electricity on light is similarly illustrated by the rotation of the plane of polarization. Sir John Herschel was the first who tried to rotate the plane of polarization of a ray of light by surrounding it with a spiral wire electrized by the great battery of two enormous plates of copper and zinc at the London Institution, but he obtained no evidence of any such action. Long afterward Professor Faraday succeeded by sending a ray of light through a piece of silico-borate of lead, which formed the core of a magnetic helix. The silico-borate took on a quasi-crystallised state during the passage of the electric current round it, giving it for the moment the property of circular polarization, analogous to that of glass in a state of tension or compression. Substances vary exceedingly in the facility with which they transmit electricity; even the same substance under another form differs remarkably in that property: charcoal, which next to the metals is the best conductor known, when under the form of diamond is quite impervious to electricity. In general, substances that are the best conductors of heat are also the best conductors of electricity, as for example the metals, which however, possess the transmissive property in very different degrees. Silver and copper are the best conductors, lead one of the worst; its resistance to the passage of electricity is twelve times greater than that of silver and copper, consequently it becomes twelve times as hot, for when a current of electricity is impeded it is changed into heat. So great is the resistance offered by a fine platinum wire, that the heat amounts to 3280° and the wire is melted, a striking instance of the correlation of electricity and heat, and of the power of the cohesive force. When electricity is passing through conducting substances or when it is static, it induces an electric state in bodies at a distance by transmission through non-conducting substances or air, for it gives polarity and tension to the adjacent atoms, and these to the next, and the next in succession, throughout the whole intervening mass,—a strong proof of the individuality and polarity of the atoms of matter.