April, 1930 
The Queensland Naturalist 
63 
Briefly, Roentgen Rays are produced by passing a 
high voltage electric current through a vacuum tube. If 
an electric discharge takes place m the air it is always 
accompanied by a sound, the air becoming incandescent 
owing to the very great resistance to the flow of the cur- 
rent, and naturally it follows that great expansion of the 
air (which now has become a good conductor) takes 
place. In a fraction of time the electrical balance is 
again established and the heated air cools rapidly and 
causes a partial vacuum. The in-rush of air to fill up the 
difference in pressure is so sudden that a crackling noise 
is heard. Thunder, accompanying lightning is an example. 
We know that the air offers a great resistance to 
the flow of electricity ; however, if we reduce the atmo- 
spheric pressure to 1 c.m. of Mercury, or lower, a silent 
electric discharge takes place; the glass container which 
we call a vacuum tube begins to glow from red to violet 
when excited by a high voltage current. As the pressure 
is further reduced the tube becomes harder, a thin stream 
of bluish colours will be noticed between the two metal 
poles. This is called the Cathode Ray, and is simply a 
path of conductance. Cathode Rays are not X-Rays, as 
they can be influenced by magnetic lines of force. The 
rarified air becomes ionised, and free electrons are bom- 
barded silently at the metal terminals within the tube. 
Roentgen Rays originate in any region where the 
velocity of electrons is suddenly changed. In an X-Ray 
tube the high speed electrons are stopped in their flight 
by the interposition of a piece of metal called a target. 
It is here where the impact of electrons takes effect. 
The problem of X-Ray production resolves itself in 
the separation of electrons from atoms, by giving them 
high speed and stopping them with sufficient suddenness. 
It will take too long to explain the ordinary gas tube, its 
construction and operation. We have several specimen 
tubes on the table which can be inspected afterwards. The 
working conditions of a gas tube is always more or less 
unstable, owing to the continually changing vacuum pres- 
sure. When the tube gets softer a greater amount of cur- 
rent will flow, and the voltage of electric pressure drops 
with a resulting loss in X-Ray penetration. On the other 
hand, a gas tube may get harder (that is the vacuum 
is lowered), and practically no current can be put through 
such a tube, and there is a great risk, if the voltage is 
increased to overcome the increased resistance, to punc- 
ture the tube by a spark and render it useless. 
The gas tube is now generally replaced by the elec- 
tron tube, and all modern X-Ray installations are fitted 
