St Ignatius College - Ignatian Yearbook (San Francisco, CA)

Page 30 text:

24 THE IGLVATIAN ciples of physics a11d chemistry as they were understood be- fore the discovery of its remarkable properties. IIISTORY or 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 inter- esting chapter in the progress of science. The chapter opens in 1895, with the discovery of the X-rays by ROHTSQH- When an electric discharge is sent through a highly evacuated tube provided with sealed-in electrodes, there is given off from the ca.thode a characteristic discharge called cathode rays. VVhen 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 sulnde or barium platinic cyanide. Another property they possess is the power of affecting sensitized photographic plates in the same way as lightg and a third very remarkable property is their power of making the air, or other gases. through which they pass, and which, under ordinary circum- stances, are practically perfect insulators, capable of convey- ing 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 dis- covery 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 Rontgeu Rays was possible, that is, considering that these rays pro- duce fluorescence, whether a iiuorescent substance could pro-

Page 29 text:

RADIO-ACTIVE SUBSTANCES 23 The present day development of Radio-Activity arose out of a discovery in 1896 by M. Henri Becquerel, that certain kinds of matter have the property of emitting a. new and pe- culiar type of radiation continuously and spontaneously, and this class of bodies has been termed radio-active. This discovery was, however, directly connected with previous dis- coveries by Crookes, Lenard and Rontgcn 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 dis- covered-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 radio- activity 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 rapid- ity and definiteness of the progress is to be found. Radio-activity has passed from the position of a descrip- tive to that of an independent science, based upon the prin-

Page 31 text:

RADIO-ACTIVE SFBSTANCES 25 duce the Rontgen Rays. Acting on this idea, he examined some iiuoreseent 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 Huoresce. While waiting for a. sunny day, the apparatus was placed in a dark drawer. Two weeks later M. Becquerel 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 Mme. Curie, one of Professor Bet-querel's students made such great progress in this new science. She found that radio-activity is an atomic propertyg that thorium acted like uraniumg that pitchblende carrying a given' weight of uranium had approximately four times greater activity than any pure uranium salt containing the same weight of ura- nium. This latter disclosure caused Mme. Curie to conclude that pitchblende contained another element 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, stron- tium, and calcium group. After the calcium and strontium had been eliminated, the barium that remained still showed strong activity. This caused Mme. Curie to investigate fur- ther, 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 ELEMENTS. The elements uranium, thorium, radium, polonium and actinium are radio-active, and by that is meant that they