Radium, action on organic matter, etc., 33 amount in pitchblende, 5 discovery of, 5 emanation, 22 energy evolved by, 34 properties of, 5, 32 Rays, alpha, 15, 16, 26 beta, 15, 16 gamma, 15, 16 identification of, 16, 25 magnetic deflection of, 14 photographing track of, 10 types of, 14 Rubidium, radio-activity of, 3 Spinthariscope, 13 Stopping power of substances, 39 Thorium X, discovery of, 18, 21 Uranium atom, disintegration of, 36 Uranium minerals, radio-activity of, 3 Uranium X, discovery of, 17, 21, 23 X-ray spectra, 44 Zinc sulphide screen, 13 TRANSCRIBER'S NOTES 1. Passages in italics are surrounded by _underscores_. 2. Images have been moved from the middle of a paragraph to the closest paragraph break. 3. The original text includes certain Greek alphabets. For this text version [alpha], [beta], [gamma] indicate first three letters of Greek alphabet respectively. 4. In this version, the number following carat character ^ is to be interpreted as follows. The expression 10^{-2} means multiplying by 0.01; 10^{10} means multiplying by 10,000,000,000. 5. In this version, the subscripted text has been replaced by an underline character _ followed by the same with curly braces { and }. For example, X_{1} indicates X with subscript 1. 6. The fractions are indicated with the help of forward character /. For example, 1/4 indicates one-fourth. 7. Other than the changes listed above, the original text has been reproduced as such.