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Radiation is the propagation of ethereal waves. If these waves are caused by comparatively slow vibrations, they can only be detected by electrical means. As the speed of vibration becomes quicker they become evident as heat; and when the speed has been further increased they can be perceived by the eye as light. A condenser whose discharge is oscillatory sets up ethereal vibrations which are essentially the same as heat or light vibrations. A condenser of large capacity may start waves a thousand miles long, but on discharging a Leyden jar of minute dimensions waves only a few feet in length would result; and if we were able to obtain a circuit so small that its dimensions were comparable with those of an atom, then without doubt waves 25-millionths of an inch long would be produced and be able to affect our eyes; they would in fact be light-waves. Electric disturbances and light both travel at the same speed in space, the velocity being 300,000 kilometres per second. In matter this equality of speed is not always obtained, for the speed of waves of different wave-lengths is retarded differently. We know that the shorter waves of light, i.e. those at the blue end of the spectrum (q.v.), are retarded more than longer ones at the red end, although the latter are only about twice as long as the former, so that we should expect some difference when we are dealing with waves many millions of times greater. It has been calculated that if a Leyden jar could be obtained to discharge an atomic charge, the wave-length of the oscillations would be that corresponding to the invisible rays at the violet end of the spectrum; and it may be due to this connection that the ultra-violet rays have so great a chemical action. Molecules are often considered to consist of electrically-charged atoms; when heated they vibrate mechanically, and thus also excite an electric radiation. Many of their different rates of vibration give rise to light, and the science of spectroscopy deals with the measurement of these different rates. Imagine a system of waves meeting a substance whose insulation is perfect; some of the radiation will be reflected and some transmitted, but none will be lost. If, however, it meet a conductor, no radiation will be transmitted; it will all be either reflected or lost. A conductor of electricity will therefore be opaque to light; this is exemplified by the fact that metals - which are good conductors - are opaque even when extremely thin. On the other hand, a transparent body must be an insulator. A perfect conductor would totally reflect radiations of every wave-length, and no dissipation would occur. In an ordinary conductor some of the radiation is destroyed; the vibrations of the ether are converted into vibrations of the atoms, and heat is produced. Radiation can be affected by matter in different ways; it can be reflected or refracted, undergo diffraction, dispersion, or absorption, or be polarised. We are accustomed to the idea that light and heat are not different in essence; but the conception that electrical disturbances are also the same in kind is not quite so general, and it was left for Mnxwell to show that light is produced by electrical vibration.

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