In Lenard's original experiments on the passage of cathode rays through windows of thin aluminium foil, the potential fall was of the order of 30,00o volts. The issuing rays, easily detected by the luminosity which they produce in a screen covered with a suitable phosphorescent substance (e.g., barium platinocyanide), traverse a distance of the order of a centimetre of air at atmospheric pressure before becoming too weak to be observed. Of recent years cathode rays of very much higher velocity, and consequently much longer range, have been produced. There are many experi mental difficulties in the way of constructing a tube that will stand up to ioo,000 volts and more, but these have been overcome by W. D. Coolidge. In his tube the electrons proceed from a hot-wire cathode, of the type used in the Coolidge X-ray tube (see X-RAYS, NATURE OF: Röntgen Rays), in a hemispherical focussing cup, and issue into the air through a window of thin nickel foil, supported by a grid of molybdenum against the pres sure of the atmosphere. The window is also the anode, and special precautions have to be taken to preserve the glass near it from puncture. Potentials as high as 200,000 volts have been used by Coolidge. With such a voltage the cathode rays can be detected at a distance of more than 4o cms. from the window in atmos pheric air. Such rays are far inferior in speed to the 13-rays of radium, but are produced by the tube in quantities far exceeding anything remotely possible with the quantities of radioactive sub stances available. The effects of this intense high speed cathode radiation are very remarkable. The rays produce a purple glow in the air round the window; they produce in calcite a phosphor escent glow which remains visible for hours after the exposure. The action on organic tissue of all kinds is very striking. Plant leaves after exposure dry up. The effects on animal tissue, as exemplified by a rabbit's ear, vary with the length of exposure. A very short exposure produces a tanning of the skin; an exposure of a second leads to the formation of a scab, which afterwards falls off with the hair, and is followed by a profuse growth of new hair which is, however, snow-white: an exposure of so sec onds leads to scabs in the raved area which leave a hole right through the ear wh-n they fall. Bacteria are killed by the radi ation, which also kills flies and beetles. Milk and butter quickly become rancid. Chemical effects have been produced in many other bodies, such as glue, gelatine and cane sugar, all of which break down under the bombardment of the swift cathode rays.
can be applied when the atoms in question cannot be obtained as a gas or a volatile compound, is to pack the anode of a discharge tube with a paste made of powdered graphite and a metallic salt : the rays then contain charged atoms of the metal in question. Such rays are known as anode rays, from their method of pro duction, but they are of the same nature as the canal rays.
In a beam of canal rays the charged atoms are mainly positive, but neutral and negatively charged atoms exist alongside. If a beam of positive atoms only be separated out by the application of a magnetic field, negative and neutral particles recreate them selves in the beam, as may be shown by a second magnetic field. This is due to the fact that an atom which starts in the beam as positively charged may become neutral by collision with an elec tron, or even negative by collision with a second electron. An equilibrium state is set up between positive, neutral and negative atoms, and, if the equilibrium is disturbed by separating out the positive ones, it establishes itself by new collisions a little further on. A moderately satisfactory theory of the reversals of charge has been worked out on the lines of the kinetic theory of gases, by considering the mean free path of charged and uncharged atoms passing through an assemblage of stationary particles. The impor tant application of the canal rays and anode rays to the accurate measurements of the atomic weights of isotopes is described in the articles POSITIVE RAYS and ISOTOPES, where the existence of negatively charged atoms and of atoms with multiple charges also receives reference.
The a-rays from radioactive bodies are of the same nature as the positive rays in discharge tubes (except in that a-rays are always helium atoms, whereas positive rays of any element can be obtained), but have very much greater energy, just as the (3-rays are of the same nature as cathode rays, but possess greater energy than can be obtained artificially. The energy of the a-particles of range 8.6 cms. from Thorium C corresponds to a fall through about ten million volts, and the energies with which a-particles are discharged from other radioactive bodies are all measured in millions of volts, as compared to the tens of thousands of volts concerned in the generation of the swiftest positively charged atoms in discharge tubes. There is thus no prospect of the generation of artificial a-rays in the laboratory. Corresponding to these voltages the velocity of the swiftest canal rays is of the order cm./sec. (for example the highest velocity obtained for hydrogen canal rays is 3 to 4X le cm./sec.) while the velocity of a-particles is of the order 2 X cm./sec. It is well known that a-particles can pass through thin metal foils (see RADIOACTIVITY, NUCLEUS) ; the smaller velocity of the canal ray atoms does not enable them to penetrate foils as thick as those accessible to the a-rays, but nevertheless it has been demonstrated that they can penetrate matter. For instance swift canal rays of hydrogen and helium penetrate aluminium foil 3.8X cm. thick and mica 2 to 6X cm. thick, and hydrogen rays have been observed to go through gold foil of thickness from 7 to 2I X cm.