The y rays are of an extraordinary pene trating character, readily passing through sev eral inches of iron or lead and several feet of water. They are not deflected by a magnetic or electric field and are a type of very pene trating The latter, consist of pulses in. the ether, in some respects to the very short waves of ultra-violet light.
If a magnetic field is applied to a pencil of rays from radium, the rays are sorted out into the three kinds. This is shown in where the magnetic field is applied at right angles to the plane of the paper. The a particles are bent to the left, the /9 to the right, while they rays proceed in a straight line without deviation. Compared with the rays, the amount of bend ing of the a rays is much exaggerated in the figure, for a very strong magnetic field is re quired to appreciably deflect the latter. The P rays consist of particles projected with different velocities, so that some are more easily beat by a magnetic field than others, This is shown in the figure.
The expulsion of a and particles from the atoms of the radioactive bodies appears to re-• stilt from an explosion in sonic of the atoms of these substances. About thirty-six thousand million of these a particles are expelled from one gram of radium element every second; and yet the number of atoms in a gram of radium is so enormous that this rate of expulsion can continue for several years without any sensible alteration or diminution of the quantity of radium.
The most important property of the a, and. y rays is their power of discharging an electri fied body and this property has been utilized as a means of quantitative measurements of the radiations. The ordinary gold leaf electro scope, such as is shown in Fig. 3, can be used to measure radiations of extremely small in tensity. A simple gold leaf is attached to a metal rod passing through an insulating cork of sulphur or ebonite. On giving a charge to this rod, the• gold leaf diverges and, under or dinary conditions, the gold leaf collapses very slowly. If, however, a radioactive substance is brought near it the charge on the system is dissipated and the gold leaf rapidly collapses.
The radiations produce a large number of positively and negatively electrified particles or ions, as they are called from the neutral gas. If the electroscope is charged positively, for example, the negative ions are drawn into it and dissipate its charge. Under suitable con ditions, the rate of movement of the gold leaf is a measure of the intensity of the radiations. A modified form of gold leaf electroscope is capable of detecting the presence of an extraor dinarily small quantity of radioactive matter.
One millionth of a gram of radium bromide causes the gold leaf to collapse in a few seconds, and, with care, the effect of one million mil lionth of a gram can be accurately determined. As a means of detection of radioactive matter in small quantity, the electroscope far trans cends in delicacy even the spectroscope. The ordinary quadrant electrometer is also a very convenient instrument for measuring the small ionization produced by the radiations in the air or other has through which they pass.
In addition to their power of giving out the three kinds of rays, radium, thorium and ium also possess another very remarkable prop-, erty. Each of these substances continuously gives off into the air a minute quantity of gas, which is itself radioactive. This ((emanation,' first discovered by Rutherford in thorium, gradually diffuses from the radioactive body into the surrounding as and can be carried away with a current of air. The aemanations,) however, only keep their power of radiating for a short time.
The activity of the emanation decreases in a geometrical progression with the time; the emanation from thorium loses half of its activ ity in about one minute; from radium in about four days; and from actinium in about four seconds.
These emanations have been found to pos sess all the properties of radioactive gases mixed in minute quantity with the air. The emanations of thorium and radium behave like heavy gases and are chemically inert like the recently discovered members of the argon helium family. The emanations can be removed from the air by the action of extreme cold. The emanation of thorium condenses at-120° C., and that from radium at —150° C. This property of condensation of the emanation from a very active substance like radium can readily be followed by the eye. The emanation re leased from radium is stored with air in a small reservoir (Fig. 4). It is slowly passed through a U tube of filled with fragments of the phosphorescent mineral willemite. The tube is immersed in a vessel filled with liquid air. The emanation condenses in the tube immediately below the level of the liquid air and the radia tions from the condensed emanation cause the willemite to become luminous. On removal of the tube and closing the ends, the emanation, after some time, is again volatilized and dis tributes itself throughout the tube, causing a uniform luminosity throughout' it. With a large quantity of radium emanation, such an experiment is extremely striking.