CORPUSCULAR RADIATIONS The more detailed properties of the different classes of corpus cular rays are dealt with in other articles, cited as occasion arises. What is here given is a general review of the various types of rays and their relationship to one another.
The corpuscular radiations may be divided into the following groups: (a) Radiations consisting of streams of electrons, often referred to generically as cathode rays, since they were first observed coming from the cathode in an exhausted tube. In this group are included the 0-rays from radioactive bodies. (b) Radi ations consisting of streams of charged atoms or charged mole cules, often called canal rays, since they were first observed passing through holes, or canals, in the cathode. These atoms are generally positively charged, and are sometimes referred to as positive rays, but since negative and neutral atoms are found in conjunction with the positive the name is not altogether suit able, although retained from long usage. The a-rays from radio active bodies fall within this group. (c) Radiations consisting of streams of uncharged atoms of low velocity, obtained by evapo rating an element in a vacuum so high that the free path of the escaping atoms is very large. Such rays are known as atomic rays.
Class (c) can clearly be included in class (b) if desired: the distinction is here made because the mode of generation of the rays and the state of charge are different. The characteristic feature of classes (b) and (c) is that the corpuscles which con stitute the radiation have the mass of atoms, which is many thousand times the mass of the electron. Andrade has suggested calling this class of rays mass rays, a term which makes no reference to the state of charge, and this term has been adopted by Aston.
Electronic Radiations.—The cathode rays were discovered by Hittorf in 1869 in the course of his work on the discharge in an exhausted tube. He recognized that the rays streamed in a straight line from the cathode, and discovered that they were deflected by a magnet, behaving in this respect "like a simple current." In 1892 Lenard showed that the rays could pass through very thin metal foils, and so succeeded in obtaining them outside the tube in which they were generated. Later it was proved by J. J. Thomson and others that the rays consisted of streams of electrons, not associated with matter in the ordinary sense, moving with a velocity governed by the potential differ ence applied to the tube. This work, and the properties of cathode rays in general, is treated in the article ELECTRICITY, CONDUC TION OF : GAS.
The velocity of cathode rays is often expressed in volts, it being understood by this that the velocity in question would be acquired by an electron moving freely through a potential differ ence of the number of volts specified. Very small velocities can be obtained by releasing electrons from a metal plate by illumi nating it with violet or ultra-violet light (see PHOTOELECTRIC ITY), or from a metal wire or strip by heating it (see THERMI omcs), and applying small accelerating or retarding fields. Ramsauer has obtained, with the help of a magnetic field and a system of screens, homogeneous beams of electrons of velocity as low as i volt (6 X At the other end of the scale we have the /3-rays from radium C which have velocities up to •998 that of light. This is a velocity of 2.994X 'ow cm./sec., and would need a field of 7.5 million volts to produce it arti ficially. Cathode rays of every velocity between these two limits are known in the laboratory, the region of velocities occupied by the spontaneous 13-rad.ations from radioactive bodies overlapping with an ample margin the upper limit of velocity producible in tubes by an artificially applied potential difference.
The difference in absorption is one of the most striking dis tinctions between cathode rays of different speed. For very slow electrons the absorbing cross-section of the atom is of the same order as the size of the atom determined by the accepted methods. Ramsauer and his school have shown very important abnormali ties of absorption for electrons whose velocity is in the neigh bourhood of a few volts, a comparatively high absorption here diminishing to an abnormally low one as the velocity drops to a volt or so, but these results do not affect the general order of the absorbing cross-section. As the velocity of the electrons increases the absorption becomes less and less : for velocities of 25, 2,56o, 24,68o, 261,50o and 662,000 volts the relative coefficients of ab sorption are as 18,000,000, 800,000, 2,900, 19 and 6. This means that the very swift electrons can pass freely through practically the whole cross-section of an atom, meeting less and less obstruc tion in so doing the higher their velocity. For the rays of highest velocity, which are not, of course, obtained artificially from a tube, but are a-rays, the cross-sectional area of an atom which acts so as to stop the electron is of the order of a millionth of the whole cross-section.