pha]
\/
Actinium D (7.4 minutes) -> [beta]&[gamma]
\/
Actinium E (unknown)

Thorium (4 x 10^{10} years?) 232.4 -> [alpha](?)
\/
Mesothorium_{1} (7.9 years)
\/
Mesothorium_{2} (8.9 hours) -> [beta]&[gamma]
\/
Radiothorium (2.91 years?) -> [alpha]
\/
Thorium X (5.35 days) -> [alpha]
\/
Emanation (76 seconds) -> [alpha]
\/
Thorium A (0.203 second) -> [alpha]
\/
Thorium B (15.3 hours) -> ([beta])
\/
Thorium C_{1} (79 minutes) { -> [alpha]
{ \/
Thorium C_{2} (?) { -> [alpha]
\/
Thorium D (4.5 minutes) -> [beta]&[gamma]
\/
Thorium E (unknown)

FIG. 6.--DISINTEGRATION SERIES FOR URANIUM, ACTINIUM, AND
THORIUM, AS GIVEN BY SODDY.]

CHAPTER IV

NATURE OF THE ALPHA PARTICLE

Disintegration of the Elements

The remarkable disintegrations related in the last chapter, in which
the heaviest known elementary atom--that of uranium (at. wt. 238)--is
by successive stages changed into others of lower atomic weight,
afford a clue to the nature of the atom and to that goal of the
chemist, the final constitution of matter. The composite nature of the
atom and some sort of interrelation of the elements had previously
been made apparent from a study of the Periodic System and data
gathered still earlier, but all attempts at working out a so-called
genesis of the elements had proved vague and unsatisfactory.

Identification of the Rays

To get an understanding of the disintegration occurring in
radio-active substances, the nature of the rays produced must be
known. These rays are the cause of the activity and their emission
accompanies the changes or disintegration. They have for the sake of
convenience been called the alpha, beta, and gamma rays. The gamma
rays have been identified with the