1915] Experiments on Slow Cathode Rays 407 



show considerable approach to each other in their properties. Both 

 kinds of rays ionise gases when they pass through them, while light 

 in the visible region of the spectrum does not do so. Unfortunately, 

 too, for the experimenter, both are so readily absorbed that they are 

 unable to penetrate even very thin layers of air, so that it is a matter 

 of very considerable difficulty to investigate their properties. Both 

 kinds of rays produced strong chemical and physiological effects when 

 their energy is absorbed in the medium through which they are 

 passing. Thus both, as I can easily show by holding starch paper in 

 their neighbourhood, produce ozone : both, too, are disastrous to some 

 of the lower organisms. Indeed, some of the latter under their 

 influence undergo the most disconcerting experience an aquatic 

 creature can suffer, for they gradually, when exposed to these rays, 

 dissolve away in the water in which they are living. The study of 

 the rays in this gap may thus become of high biological and medical 

 importance. 



The method I have used to produce rays between Schumann rays 

 and ordinary Rontgen rays is to bombard a metal target with very 

 slow cathode rays. The cathode rays which produce the Rontgen rays 

 in an ordinary Rontgen ray bulb have speeds corresponding to 20,000 

 volts or so and upwards ; the cathode rays I have used have speeds 

 corresponding to from 5 to 2000 volts. It is well known that the 

 greater the speed of the cathode particle the harder the Rontgen rays 

 they produce. I can show this very distinctly by using a special and 

 very convenient form of Rontgen ray bulb — the Coolidge tube — for 

 the loan of which I am indebted to Messrs. Cossar. The essential 

 feature of this tube is that the cathode produces its own supply of 

 cathode particle, as it consists of a tungsten filament heated to 

 redness by an independent heating circuit. The white hot tungsten 

 gives out cathode rays which have no velocity when they start from 

 the cathode ; the velocity is communicated by the potential difference 

 between the cathode and anode. As this is increased the velocity 

 acquired by the particles before they strike against the target is also 

 increased. The number of cathode particles does not depend upon 

 this potential difference, but is determined solely by the temperature 

 of the tungsten spiral. The vacuum in the bulb is an exceedingly 

 good one, so good that you do not see the ordinary green phosphor- 

 escence on the glass when the bulb is in action. If I hold my hand 

 in front of a platino-cyanide screen, and gradually increase the speed 

 of the cathode particles by increasing the potential difference, you 

 will see that at low speeds very few Rontgen rays reach the screen 

 through my hand ; as I increase the speed some get through the 

 flesh, but are stopped by the bones, and at the very highest speeds an 

 appreciable amount gets through the bones themselves. 



I will now pass on to consider the production of soft Rontgen 

 rays by the impact of slow cathode rays. The first method was as 

 follows : — Fisf. l is a Wehnelt cathode ; the anode A is a brass rod 



