University of San Francisco - USF Don Yearbook (San Francisco, CA)

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24 TIIE I (i ATI AS ciplcs of physics and chemistry as they were understood before the discovery of its remarkable properties. History of Radio-Activity. Our present understanding of radio-activity has been the result of many successive discoveries, and the historical order in which this result has been achieved is a very interesting chapter in the progress of science. The chapter opens in 1895, with the discovery of the X-rays by Rontgen. When an electric discharge is sent through a highly evacuated tube provided with sealed-in electrodes, there is given off from the cathode a characteristic discharge called cathode rays. When the electrons of the cathode stream are suddenly stopped by hitting a target within the evacuated tube, a different sort of rays is given off from the target, and these are called X-rays. They are invisible, but. like the cathode rays inside the tube, their presence is manifested by their power to cause strong fluorescence when they impinge upon certain substances like zinc sulfide or barium plat inic cyanide. Another property they possess is the power of affecting sensitized photographic plates in the same way as light; and a third very remarkable property is their power of making the air, or other gases, through which they pass, and which, under ordinary circumstances. are practically perfect insulators, capable of conveying limited quantities of both positive and negative electricity. This process is known as ionisation, and the rays are said to ionise the gases, namely, to make them for the time being partial conductors of electricity. In 1896, with the awakening that followed the researches of Rontgen to the existence of new types of radiations of a character utterly different from those of light, came the discovery of the property of radio-activity by M. Becquerel. In his investigations of the X-rays. M. Becquerel was partially interested in their power of fluorescence, and it was his desire to find whether an inversion of the phenomena of the Rontgen Rays was possible, that is. considering that these rays produce fluorescence, whether a fluorescent substance could pro-

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RADIO ACTIVE SVB8TANCE8 23 The present day development of Radio-Activity arose out of a discovery in 189G by M. Henri Becquerel, that certain kinds of matter have the property of emitting a new and peculiar type of radiation continuously and spontaneously, and this class of bodies has been termed 44radio-active. This discovery was, however, directly connected with previous discoveries by Crookes, Lenard and Rontgen of other new kinds of radiation. The discovery of this new property of self-radiance. or '‘radio-activity,” has proved to be the beginning of a new science, in the development of which physics and chemistry have worked together in harmony. The pioneer in the chemical development of the subject was Madame Curie, who, by the discovery of radium, extended our knowledge of the new property out of the region of the infinitely small effects in which it had its beginning, and demonstrated it on a scale that could neither be explained nor explained away. On the physical side, the brilliant and elaborate researches of Professor Rutherford, at first mainly with thorium—an element which, like uranium, is so feebly active that it had been studied for a century before its radio-activity was discovered—paved the way for a complete and general theory of the cause and nature of the new property. According to this theory the elements exhibiting radioactivity are in the process of evolution into lighter and more stable forms, and the radiations spontaneously emitted are due to the incessant flight, radially from the substance, of a swarm of light fragments of the original atoms, expelled in the course of their explosive disintegration. This theory has recently received a direct experimental confirmation in the discovery of the continuous production of the element helium from radium. In these advances, physics and chemistry have borne equal shares, and in the close relation between the two sciences throughout the investigations the secret of the rapidity and definiteness of the progress is to be found. Radio-activity has passed from the position of a descriptive to that of an independent science, based upon the prin-

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It A DIO-ACT! YE 8VB8TASCES 25 duce the Rout gen Rays. Acting on this idea, he examined some fluorescent compounds of uranium. His method was to place the bare salt above a photographic plate, which was carefully wrapped up in opaque material, and so protected completely from the direct action of light, and to expose the salt to direct sunlight, so as to cause it to fluoresce. While waiting for a sunny day, the apparatus was placed in a dark drawer. Two weeks later M. Bccqucrel decided to test the freshness of the plate by developing it, and to his surprise, a dark spot appeared on it, beneath where the salt had rested. This was proof conclusive that some unsuspected rays had passed through the opaque material, and after delicate tests had been made, it was shown beyond doubt that the new and unsuspected rays were really electrical. It was at this stage that Mine. Curie, one of Professor Becquerers students made such great progress in this new science. She found that radio-activity is an atomic property; that thorium acted like uranium; that pitchblende carrying a given weight of uranium had approximately four times greater activity than any pure uranium salt containing the same weight of uranium. This latter disclosure caused Mine. Curie to conclude that pitchblende contained another clement that was also radio-active, and she proceeded to prove this conclusion. The pitchblende was dissolved, and the various elements in it were precipitated and tested for radio-activity. The principal activity was found to be concentrated in the barium, strontium, and calcium group. After the calcium and strontium had been eliminated, the barium that remained still showed strong activity. This caused Mine. Curie to investigate further, and the element radium was eventually separated by the fractional crystallization of its salts from the corresponding barium salts. This discovery of the new element, radium, was made in 1898. Radio-Active Eli:ments. The elements uranium, thorium, radium, polonium and actinium are radio-active; and by that is meant that they