SCIENCE AND INDUSTRY. 
aR . A Brief Sketch of their History, 
A-Rays: Nature, and Technical Applications. 
By PROFESSOR KERR GRANT, University of Adelaide. 
The actual discovery of X-rays was made by William Réntgen, Professor 
of Physics in the University of Wurzburg, in the year 1895; a discovery not 
to be regarded as an isolated peak of scientific achievement but as the 
culmination of a long series of investigations into the phenomena accom- 
panying the discharge of electricity through rarefied gases by many previous 
investigators. 
Chief among the names of these earlier investigators are those of Faraday, 
Crookes, and Schuster in England; of Hittorf, Goldstein, and Lenard in 
Germany. 
The nature of these antecedent discoveries may be briefly summarized : 
Air, or any other gas, at atmospheric pressure, and in its ordinary condition, 
does not permit of the passage of an electric discharge from an electrified 
conductor to an adjacent one unless the potential difference between the 
conductors amounts to some 75,000 volts for each inch of space separating - 
them; if this voltage gradient be exceeded a discharge takes place in the 
_ form of an electric spark or series of sparks. If, Towayert the air or other 
gas is contained in a glass tube fitted with metal electrodes, and if the pressure 
of the gas within this tube be steadily lowered by means of any form of air- 
pump, the discharge is found to pass more and more readily, the necessary 
voltage falling in proportion to the decrease of pressure. Moreover, the 
character of the discharge changes greatly as rarefaction increases, the violent 
intermittent spark being gradually replaced by a silent continuous flow of 
electricity, the path of which is no longer confined to an extremely narrow 
band of air but includes, at pressures of 1 inch or less, all the gas between 
_ the electrodes, as is manifest by the luminosity displayed by the contents 
of the tube. 
This luminosity, however, is not uniform along the length of the tube. 
The negative electrode; or kathode, is enveloped in a luminous mantle; this 
is succeeded by a space devoid of luminosity, the Faraday dark space, and 
this by a luminous column, sometimes striated in appearance, extending to 
the positive terminal or anode. 
This is the géneral character of the appearance with pressures between 
1 centimetre and 1 millimetre of mercury; with further reduction another 
dark space appears between the negative glow surrounding the kathode 
and the kathode itself. This is known as the “ Crookes dark space.” 
As the vacuum becomes higher this latter dark space extends further and 
further outwards from the kathode, until it extends laterally to the walls of 
the tube and to the anode itself. Corresponding with this extension the 
* luminosity of the gas falls off and becomes restricted to a beam or pencil 
proceeding outward from the kathode and suggestive in appearance of the 
track of a beam of sunlight through the dusty air of a darkened room. The 
colour of this pencil depends upon the nature of the rarefied gas; where it 
strikes the wall of the tube a bright green fluorescence of the glass is pro- 
duced. As Crookes first showed, it carries sufficient energy to heat pieces of 
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