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Radioactivity

rays, substance, radium, active, particles, ordinary and uranium

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RADIOACTIVITY. The discovery that uranium possesses the Property of spontane ously and continuously emitting penetrating rays, capable of passing through bodies opaque to ordinary light, was made by Becquerel in 1896. Since that time our knowledge of the subject of radioactivity has increased with great rapidity and a very large mass of experimental facts has now been accumulated. Only a very, brief account of the_remarkable properties posi sessed by the radioactive bodies is possible ki this short article.

Shortly after the discovery of this property in uranium, Mme. Curie and Schmidt inde pendently found that the element thorium, which is used in the manufacture of Welsbach mantles, was also radioactive and to about the same degree as uranium. The latter substance is the chief constituent of the mineral pitch blende and in an examination of the radioactive properties of this mineral, Mme. Curie was led to the conclusion that other and more radio active substance must be present in it. By an admirable piece of chemical research Mme. and P. Curie succeeded in separating from it two new very- active substances which were called' radium and polonium. The substance radium was well named as, in the pure state, it gives out radiations about 4,000,000 times as intense as uranium. .Polonium has not yet been chemi cally isolated, It differs from radium in losing its activity with time. Several years after separation the activity has diminished gradu ally to an insignificantly small fraction of its original value.

In addition to these bodies, Debierne and Giesel independently discovered another very active substance called actinium, while Hof mann found that lead separated from pitch blende was active and called the radioactive substance present in it "radio-lead)) Somewhat later a radioactive substance called ionium was isolated from uranium minerals by Boltwood. With the exception of radium, none of these bodies have been chemically isolated.

The radiations from these substances are in visible to the eye, but possess the properties of acting on a photographic plate and discharg ing an electrified body. The rays from a very

active substance like radium cause many phos phorescent substances to become luminous. An ordinary X-ray screen, crystalline zinc sulphide and the minerals willemite and kunzite glow brightly when exposed to radium rays. A pho tographic plate is rapidly affected by bringing near a few milligrams of radium bromide, and an electroscope quickly loses its charge.

The radiations from the active bodies have been shown to consist of three distinct kinds, known as a, F3, and y rays. Unlike ordinary light-waves, the a and [3 rays consist of a flight of charged particles projected with enormous velocity. They differ from ordinary light in asmuch as a pencil of these rays is deflected by a magnetic and electric field. These properties of the radiations have allowed us to determine the size of the particle and its velocity of pro jection. Rutherford found that the a rays con sisted of atoms of matter carrying a positive charge and moving with a velocity of about 20,000 miles a second. The a rays arc readily stopped by matter, being absorbed by a few centimeters of air and by a sheet of ordinary note paper. The /3 particles, on the other hand, are far more penetrating and pass through sev eral millimeters of aluminuni or iron. The 0 particles differ from the a particles in carrying• a negative charge and in their much greater velocity of projettian. Sonic of them move with a velocity of over 170,000 miles per sec ond— a speed very nearly equal to that of light. The particle has been found by Becquerel and Kaufmann to be identical with the cathode particle produced by a discharge in a vacuum tube. It has the smallest size of any body known to science, for its mass is only about 1/1700 of that of the atom of hydrogen. It was shown in a series of experiments conducted by Rutherford that the a particles consist of atoms of helium bearing a positive charge. The relative sizes and velocities of the a, and Q are diagrammatically. shown in Fig. 1. The relative energies of motion of the particles are also shown.

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