[Footnote 31:  The whole of this argument is brilliantly set forth by Professor Lorentz in a lecture delivered to the Electrotechnikerverein at Berlin in December 1904, and reprinted, with additions, in the Archives Neerlandaises of 1906.—­ED.]

In the same way Professor Hesehus has explained how contact electrification is produced, by the tendency of bodies to equalise their superficial properties by means of a transport of electrons, and Mr Jeans has shown that we should discover the existence of the well-known laws of distribution over conducting bodies in electrostatic equilibrium.  A metal can, in fact, be electrified, that is to say, may possess an excess of positive or negative electrons which cannot easily leave it in ordinary conditions.  To cause them to do so would need an appreciable amount of work, on account of the enormous difference of the specific inductive capacities 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.