THE URANIUM-238 SERIES Element Symbol Radiation Half-life Uranium ²³⁸U α 4.51 × 10⁹ years Thorium ²³⁴Th β 24.1 days Protactinium[17] ²³⁴Pa β 1.18 minutes Uranium ²³⁴U α 2.48 × 10⁵ years Thorium ²³⁰Th α 7.6 × 10⁴ years Radium ²²⁶Ra α 1.62 × 10³ years Radon ²²²Rn α 3.82 days Polonium[18] ²¹⁸Po α (99.98%) and β 3.05 minutes (0.02%) Lead ²¹⁴Pb β 26.8 minutes Astatine ²¹⁸At α 1.3 seconds Bismuth[18] ²¹⁴Bi β (99.96%) and α 19.7 minutes (0.04%) Polonium ²¹⁴Po α 1.6 × 10⁻⁴ second Thallium ²¹⁰Tl β 1.32 minutes Lead ²¹⁰Pb β 22 years Bismuth[18] ²¹⁰Bi β (~100%) and α (~2 × 5.0 days 10⁻⁴%) Polonium ²¹⁰Po α 138.4 days Thallium ²⁰⁶Tl β 4.30 minutes Lead ²⁰⁶Pb ... Stable [Illustration: The Uranium-238 Series] SUGGESTED REFERENCES Books _How Old Is the Earth?_, Patrick M. Hurley, Doubleday & Company, Inc., Garden City, New York, 1959, 160 pp., $1.25. _Radiocarbon Dating_ (second edition), Willard F. Libby, University of Chicago Press, Chicago, Illinois, 1955, 175 pp., $5.00 (hardback); $1.95 (paperback). _The Birth and Death of the Sun_, George Gamow, The Viking Press, New York, 1949, 238 pp., $4.75 (out of print but available through libraries), $0.60 (paperback) from the New American Library of World Literature, Inc., New York. _Inside the Nucleus_, Irving Adler, The John Day Company, Inc., New York, 1963, 191 pp., $4.95 (hardback); $0.60 (paperback) from the New American Library of World Literature, Inc., New York. _Ages of Rocks, Planets, and Stars_, Henry Faul, McGraw-Hill Book Company, New York, 1966, 109 pp., $2.45. _Principles of Geochemistry_ (second edition), Brian Mason, John Wiley & Sons, Inc., New York, 1958, 310 pp., $7.50. (New edition due in 1967.) _Potassium Argon Dating_, J. H. Zahringer and O. A. Schaeffer, Springer-Verlag New York, Inc., New York, 1966, 250 pp., $12.50. Articles A Clock for the Ages: Potassium Argon, Garniss H. Curtis, _National Geographic Magazine_, 120: 590 (October 1961). Exploring 1,750,000 Years into Man’s Past, L. S. B. Leakey, _National Geographic Magazine_, 120: 564 (October 1961).