SCIENCE AND INDUSTRY. 
The electron manifests itself not only in the form of kathode rays, but 
in a great variety of other circumstances. Thus, the effect of a magnetic 
field upon the spectrum of the light emitted by a radiating gas, discovered 
by Zeeman in 1897, was explained by Lorentz on the hypothesis that the 
light is radiated by an electron rotating within the atom. he ratio charge : 
mass, determined spectroscopically, agrees with that found for the kathcde 
rays. The positive electrification which electro-positive metals acquired 
when radiated by light of short wave-length (or by X-rays) is due to an 
ejection of electrons by their atoms; the same electrons are spontaneously 
ejected with enormous speed by the atoms of many radio-active substances ; 
they also issue from metals when these are heated to very high temperatures ; 
' the process of gaseous ionisation has been shown to consist of the detach- 
ment or attachment of a single electron from or to the ionised atom; they 
- can be shown to be concerned in the processes of metallic conduction and of 
optical dispersion. 
In short, the electron declares its presence in every known kind of atom 
and in a great variety of conditions, and is certainly to be regarded as a 
_ universal constituent of matter. 
The speed with which these electrified corpuscles travel depends solely 
on the voltage difference between kathode and anode; in other words, 
their kinetic energy is due to the electrical work done on them. Thus, with 
100 volts difference they attain a speed of 8,400 kilometres (about 5,200 
miles) per second; with 1,000, ten times this speed, and so on. At very 
high voltages and speeds, however, it would follow both from electrical 
theory and on more general grounds that the increase of speed with voltage 
should be less than at moderate velocities. Experiment has confirmed this 
view. sal 
At high speeds the penetrating power of the kathode rays is considerable, 
Lenard, of Heidelberg, in 1893, made vacuum tubes with thin windows of 
aluminium foil, and, using high voltages, succeeded in making the rays 
penetrate these windows and emerge into the outside air. He thus was 
able to show that they exert a powerful photographic action, that they can 
- excite fluorescence, and that they ionise the air through which they pass. 
. His work is generally corisidered by German men of science as the immediate 
precursor of Réntgen’s discovery. 
X-rays are produced by the impact of the kathode rays on matter of 
any kind. (This statement is universally true only if the essential identity 
of X-rays and ordinary light be allowed.) In Réntgen’s original experiment 
it-was the walls of the bulb on which they impinged; in later types of 
tube a metal “anti-kathode”’ or “target” placed directly in the path of 
the beam of rays, which, moreover, can be focussed on this target by the 
simple device of making the kathode from which they radiate concave towards 
the target. For a given target the amount of X-radiation emitted is strictly 
proportional to the number of kathode rays which hit the target; the pene- 
trating power depends both on the speed of the rays and on the material 
of which the target is composed. : 
The properties of X-rays were very systematically and thoroughly 
investigated by Réntgen himself. In almost all respects they resemble 
_ ordinary light so strongly as to suggest an essential similarity, if not identity. 
They affect, as does light, a photographie plate ; they can excite fluorescence 
in certain materials, as can light; they ionise or render conductive air or 
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