Thorium (4 × 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 _X_ rays discovered by Röntgen and are a form of energy analogous to light. The beta rays are particles of negative electricity or electrons. With these, then, we have some degree of familiarity. But what are the alpha rays? An answer to this question should make clearer the character of the changes taking place, and should give some insight into the composition and mechanism of the atom. The Alpha Rays It has already been stated that these alpha rays are similar or analogous to the canal rays, but this advances the matter very little, as the nature of these canal rays has not been fully determined. The full identity with them, if proved, should have an important theoretical bearing. Alpha Rays Consist of Solid Particles