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apacities of the metal and of the insulating medium in which it is plunged. Electrons, however, which, on arriving at the surface of the metal, possessed a kinetic energy superior to this work, might be shot forth and would be disengaged as a vapour escapes from a liquid. Now, the number of these rapid electrons, at first very slight, increases, according to the kinetic theory, when the temperature rises, and therefore we must reckon that a wire, on being heated, gives out electrons, that is to say, loses negative electricity and sends into the surrounding media electrified centres capable of producing the phenomena of ionisation. Edison, in 1884, showed that from the filament of an incandescent lamp there escaped negative electric charges. Since then, Richardson and J.J. Thomson have examined analogous phenomena. This emission is a very general phenomenon which, no doubt, plays a considerable part in cosmic physics. Professor Arrhenius explains, for instance, the polar auroras by the action of similar corpuscules emitted by the sun. In other phenomena we seem indeed to be confronted by an emission, not of negative electrons, but of positive ions. Thus, when a wire is heated, not _in vacuo_, but in a gas, this wire begins to electrify neighbouring bodies positively. J.J. Thomson has measured the mass of these positive ions and finds it considerable, i.e. about 150 times that of an atom of hydrogen. Some are even larger, and constitute almost a real grain of dust. We here doubtless meet with the phenomena of disaggregation undergone by metals at a red heat. CHAPTER IX CATHODE RAYS AND RADIOACTIVE BODIES Sec. 1. THE CATHODE RAYS A wire traversed by an electric current is, as has just been explained, the seat of a movement of electrons. If we cut this wire, a flood of electrons, like a current of water which, at the point where a pipe bursts, flows out in abundance, will appear to spring out between the two ends of the break. If the energy of the electrons is sufficient, these electrons will in fact rush forth and be propagated in the air or in the insulating medium interposed; but the phenomena of the discharge will in general be very complex. We shall here only examine a particularly simple case, viz., that of the cathode rays; and without entering into details, we shall only note the results relating to these rays which furnish valuable arguments in favour of the electronic hypothesis and supply
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