RADIATION, RAYS. Modern physics has developed mainly as a result of an intensive study of radiations and rays, and their relation to matter. It is possible to make a distinction between the two terms, but they are generally used by men of science as more or less synonymous. We have on the one hand the vast field of electromagnetic radiations, all travelling through empty spaces with the same velocity, the velocity of ordinary light. According to the wave-length of these radiations we speak of wireless waves, infra-red radiation, visible light, ultra-violet light, X-rays or 7-rays; the order given being that of decreasing wave-length. On the other hand we have radiations which con sist iof streams of particles all moving in the same direction, either corpuscles of electricity, not associated with ordinary matter, as in cathode rays, or material particles which may be either charged or uncharged, such as positive rays or molecular rays. These particles may travel with any velocity, from zero, up to, in extreme cases, a velocity approaching that of light. We might
distinguish between 'the two classes by agreeing to confine the term radiation to electromagnetic waves, and to restrict the term rays to streams of particles, but while a tendency in this direction is traceable it is by no means correct to represent this convention as one invariably used by experts. For instance, it is quite usual in dealing with /3-rays, which are swift electrons, and thus cor puscular in nature, to speak of the scattered radiation and the transmitted radiation, and even to refer to the atoms of radio active matter thrown back by the discharge of particles as a recoil radiation. (See, e.g., Rutherford's Radioactive Substances.) On the other hand it is a commonplace to speak of rays of light. To indicate further the way in which, in actual practice, the terms are used indifferently it may be recalled that in a previous edition of the Encyclopedia electromagnetic radiation was dealt with, in different aspects, both under the heading RADIATION and under the heading RAY.