it follows that the energy of the α particle from radium is almost 83 times the energy of the β particle. If equal numbers of α and β particles are projected per second, the total energy radiated in the form of α rays is about 83 times the amount in the form of β rays. Evidence will be given later (section 253) to show that the number of α particles projected is probably four times the number of β particles; so that a still greater proportion of the energy is emitted in the form of α rays. These results thus lead to the conclusion that, from the point of view of the energy emitted, the α rays are far more important than the β rays. This conclusion is supported by other evidence which is discussed in chapters XII and XIII, where it will be shown that the α rays play by far the most important part in the changes occurring in radio-active bodies, and that the β rays only appear in the latter stages of the radio-active processes. From data based on the relative absorption and ionization of the β and γ rays in air, it can be shown that the γ rays carry off about the same amount of energy as the β rays. These conclusions are confirmed by direct measurement of the heating effect of radium, which is discussed in detail in chapter XII. Footnote 111: In an examination of uranium the writer (_Phil. Mag._ p. 116, Jan. 1899) found that the rays from uranium consist of two kinds, differing greatly in penetrating power, which were called the α and β rays. Later, it was found that similar types of rays were emitted by thorium and radium. On the discovery that very penetrating rays were given out by uranium and thorium as well as by radium, the term γ was applied to them by the writer. The word “ray” has been retained in this work, although it is now settled that the α and β rays consist of particles projected with great velocity. The term is thus used in the same sense as by Newton, who applied it in the _Principia_ to the stream of corpuscles which he believed to be responsible for the phenomenon of light. In some recent papers, the α and β rays have been called the α and β “emanations.” This nomenclature cannot fail to lead to confusion, since the term “radio-active emanation” has already been generally adopted in radio-activity as applying to the material substance which gradually _diffuses_ from thorium and radium compounds, and itself emits rays. Footnote 112: This method of illustration is due to Mme Curie, _Thèse présentée à la Faculté des Sciences de Paris_, 1903. Footnote 113: Giesel, _Annal. d. Phys._ 69, p. 834, 1899. Footnote 114: Meyer and Schweidler, _Phys. Zeit._ 1, pp. 90, 113, 1899. Footnote 115: Becquerel, _C. R._ 129, pp. 997, 1205, 1899. Footnote 116: Curie, _C. R._ 130, p. 73, 1900. Footnote 117: Rutherford, _Phil. Mag._ January, 1899. Footnote 118: Rutherford and Grier, _Phil. Mag._ September, 1902. Footnote 119: Becquerel, _C. R._ 130, pp. 206, 372, 810, 979. 1900.