The results, however, indicate that four α particles are expelled from radium in radio-active equilibrium for each β particle and thus confirm the theory of successive changes. Footnote 325: _Phil. Trans._ A. p. 25, 1901. Footnote 326: P. Curie and Laborde, _C. R._ 136, p. 673, 1903. Footnote 327: Runge and Precht, _Sitz. Ak. Wiss. Berlin_, No. 38, 1903. Footnote 328: P. Curie, Société de Physique, 1903. Footnote 329: Rutherford and Barnes, _Nature_, Oct. 29, 1903. _Phil. Mag._ Feb. 1904. Footnote 330: Paschen, _Phys. Zeit._ Sept. 15, 1904. Footnote 331: Rutherford and Barnes, _Nature_, Dec. 18, 1904; _Phil. Mag._ May, 1905. Footnote 332: Pegram, _Science_, May 27, 1904. CHAPTER XIII. RADIO-ACTIVE PROCESSES. =254. Theories of radio-activity.= In previous chapters, a detailed account has been given of the nature and properties of the radiations, and of the complex processes taking place in the radio-active substances. The numerous products arising from the radio-elements have been closely examined, and have been shown to result from a transformation of the parent element through a number of well-marked stages. In this chapter, the application of the disintegration theory to the explanation of radio-active phenomena will be considered still further, and the logical deductions to be drawn from the theory will be discussed briefly. A review will first be given of the working hypotheses which have served as a guide to the investigators in the field of radio-activity. These working theories have in many cases been modified or extended with the growth of experimental knowledge. The early experiments of Mme Curie had indicated that radio-activity was an atomic and not a molecular phenomenon. This was still further substantiated by later work, and the detection and isolation of radium from pitchblende was a brilliant verification of the truth of this hypothesis.