old minerals the transformations have been in progress for inter- vals measured by millions of years, the end-product should collect and be an invariable companion of the radioelement. Boltwood showed that lead is always present in old radioactive minerals and in amount to be expected from their uranium con- tent and geologic age.
In recent years this problem has been definitely attacked in the light of the chemical generalization already given. It was clear from this that the end-products of uranium, thorium and actinium should all be isotopes of lead but with atomic weights 206, 208 and 206 respectively. In other words, uranium-lead if uncontaminated with ordinary lead should show a smaller atomic weight than ordinary lead (207), while thorium-lead should give a higher value. By the work of Richards, Soddy and Honigschmid, these conclusions have been definitely confirmed. The lowest value for uranium-lead is 206, and the highest for thorium-lead 207-7.
Since any admixture with ordinary lead tends to give a value nearer 207, these results may be considered as a definite proof of the nature and atomic weight of the end-products. In minerals containing both uranium and thorium the atomic weight of the mix- ture of the isotopes will depend on the relative amounts of these two elements and their relative rates of transformation. In unaltered minerals the determination of the amount of lead coupled with its average atomic weight allows us to determine the amount of ura- nium-lead even if some ordinary lead be present. In this way it should be possible to make a reliable estimate of the age of selected minerals and thus indirectly the age of the geologic strata. The amount of helium in the mineral gives a minimum estimate of its age, for, except in the most compact minerals, some of the helium must undoubtedly escape.
Nature and Properties of the a. Rays (re-stated). Although the o rays from active substances are of small penetrating power compared with the /3 or 7 rays, they are responsible for most of the energy evolved by radioactive substances and contribute most of the ionization. Rutherford showed in 1903 that the o rays were deflected in a powerful magnetic and electric field and consisted of positively charged particles projected with high velocity. From the first it seemed probable that the a particle was an atom of helium and this was subsequently confirmed in a number of ways. The value of e/m the ratio of the charge on the particle to its mass and the velocity can be determined from observations on the deflection of the pencil of rays by a magnetic field and electric fields. In this way Ruther- ford and Robinson showed that the o particle, whether from the radium emanation, radium A or C, gave a value of e/m = 4820 e.m. units, while the electrochemical value of e/m = 48 26, assuming that the mass of the helium atom is 4-00 and that it carries two unit positive charges. The magnitude of the charge carried by each particle was measured by Regener and Rutherford and Geiger and found to be twice that carried by the electron. The velocity of the a particles expelled from radium C (of range 7-06 cms.) was found to be 1-92X10' cm. per second, or about Vis the velocity of light. From this result the velocity of expulsion of all a particles can be calculated from the relation found by Geiger, that V 3 =KR where V is the velocity of the particle and R its range in air. The evidence indicates that the a particles from active products are in all cases atoms of helium. The a particles from a given product are all emitted with constant velocity which is characteristic for that product. We have already mentioned that the velocity of expulsion appears to be connected with the period of transformation of the element. The laws of absorption of the a particle were first worked out by Bragg and Kleeman. On account of their great energy of motion, the a particle travels in nearly a straight line through the gas, producing intense ionization along its track. The effects produced by the a particle, whether measured by ionization, phosphores- cence or photographic action, vanish suddenly after the a particle has traversed a definite amount of matter. This definiteness of the end of the range of the a particle of given velocity is remarkable. The range of the a particle is usually expressed in terms of cms. of air traversed at 15 C. and 760 mm. pressure.
On account of its great energy of motion the effect due to a single a particle can be detected in a variety of ways. Sir William Crookes
first noted that the o rays produce scintillations when they fall on a screen of phosphorescent zinc sulphide. It is now known that each of these scintillations is due to the impact of a single a particle. The number of scintillations can be counted with the aid of a suit- able microscope, and this method has proved of great utility in many investigations. Scintillations due to o rays are observed in certain diamonds, but they are usually not so bright as in zinc sulphide. Kinoshita has shown that a single a particle produces a detectable effect on a photographic plate. When the a rays fall on a plate nearly horizontally the track of the a particle is clearly visible under a high-power microscope. By the expansion method developed by C. T. R. Wilson, the track of the a particle through the gas is made visible by the condensation of the water on each of the ions produced. In a similar way the track of a ft particle can be easily shown. The photographs of these trails bring out in a striking and concrete way not only the individual existence of o and ft par- ticles, but the main effects produced in their passage through matter.
Properties of ft andy Rays (re-stated). We have seen that the /3 particles, which are emitted by a number of radioactive products, consist of swift negative electrons spontaneously liberated during the transformation of active matter. The velocity of expulsion and the penetrating power of /3 rays vary widely for different products. For example, the rays from radium B are much more easily absorbed by matter than the swift /3 rays from radium C. Moseley showed that in the case of these two products each disintegrating atom gave rise on the average to one j3 particle.
There is undoubtedly a close connexion between ft and y rays, and swift ft rays are usually accompanied by penetrating y rays. For example, radium C, which emits very swift ft rays, some of which reach a velocity more than 0-98 of the velocity of light, gives rise to the most penetrating y rays observed in the uranium-radium series. There is one very notable exception, viz. radium E, which emits swift ft particles but weak y rays. Gray has shown that rays in passing through matter give rise to y rays, and that these in some cases correspond to the characteristic X radiations observed by Barkla. The absorption of the y rays has been determined by the electrical method. Radium B has been found to emit several groups of y rays which differ widely in penetrating power. The greater part of the rays from radium C consist of penetrating y rays which are exponentially absorbed by matter. The ionization in an elec- troscope falls off according to the equation I/Io = eMd where d is the thickness of matter traversed and /* the coefficient of absorp- tion. When lead is used as an absorbing material the value of jj=o-5 for the most penetrating y rays from radium C. The ab- sorption coefficient for different kinds of matter is roughly propor- tional to the density, indicating that the absorption depends only on the mass of matter traversed.
The general evidence indicates that the y rays consist of types of characteristic radiations which are excited by the passage of the ft rays through the electronic system of the atom, but the y rays from radium C are far more penetrating than any type of charac- teristic radiation observed in X rays generated in a vacuum tube.
Rutherford and Andrade have determined the spectrum of the y rays from radium B and C by reflection from rock-salt. The most intense lines due to radium B are identical in wave-length with the X-ray spectrum of lead. This is to be expected, since radium B is an isotope of lead. The lines due to the " K " characteristic radia- tion are also observed. General considerations, however, indicate that the wave-length of the most penetrating y rays is much too short to resolve or detect by the crystal method. In order to excite such rays in an X-ray tube potential differences of the order of two million volts will be necessary.
When the y rays from a product like radium B or radium C are bent by a magnetic field and fall on a photographic plate, a kind of magnetic spectrum is obtained. Superimposed on the continuous spectrum clue to particles of all velocities (between certain limits) certain sharp lines are observed, each of which represents a definite group of ft rays which are emitted at the same speed. The velocity corresponding to each line in the spectrum has been determined for a number of /3-ray products by Hahn and Miss Meitner. The mag- netic spectrum of radium B and radium C was examined in detail by Rutherford and Robinson and more than 50 lines were observed, representing ft particles projected over a wide range of velocity. The appearance of these lines in the spectrum appears to be connected with the emission of y rays and is believed to be due to the conver- sion of the energy of the y ray of definite frequency into the energy of an electron according to the quantum relation. When a thin layer of absorbing material is placed over the source, the primary ft rays diminish in velocity and the lines become broad and diffuse. At the same time, however, new groups of ft rays are formed by the con- version of y rays into ft rays in passing through the absorbing ma- terial, and these give well-marked bands on the photographic plate, occupying very nearly the same position as those due to the primary ft rays before absorption. Results of this kind have an important bearing on the general problem of radiation, and give us indications of the facts to be accounted for in dealing with the conversion of swift ft rays into y rays of high frequency, and vice versa.
