ABBREVIATIONS OF REFERENCES TO SOME OF THE JOURNALS. _Ber. d. deutsch. Chem. Ges._ Berichte der deutschen chemischen Gesellschaft. Berlin. _C. R._ Comptes Rendus des Séances de l’Académie des Sciences. Paris. _Chem. News._ Chemical News. London. _Drude’s Annal._ Annalen der Physik. Leipzig. _Phil. Mag._ Philosophical Magazine and Journal of Science. London. _Phil. Trans._ Philosophical Transactions of the Royal Society of London. _Phys. Rev._ Physical Review. New York. _Phys. Zeit._ Physikalische Zeitschrift. _Proc. Camb. Phil. Soc._ Proceedings of the Cambridge Philosophical Society. Cambridge. _Proc. Roy. Soc._ Proceedings of the Royal Society of London. _Thèses-Paris._ Thèses présentées à la Faculté des Sciences de l’Université de Paris. _Wied. Annal._ Annalen der Physik. Leipzig. CHAPTER I. RADIO-ACTIVE SUBSTANCES. =1. Introduction.= The close of the old and the beginning of the new century have been marked by a very rapid increase of our knowledge of that most important but comparatively little known subject—the connection between electricity and matter. No study has been more fruitful in surprises to the investigator, both from the remarkable nature of the phenomena exhibited and from the laws controlling them. The more the subject is examined, the more complex must we suppose the constitution of matter in order to explain the remarkable effects observed. While the experimental results have led to the view that the constitution of the atom itself is very complex, at the same time they have confirmed the old theory of the discontinuous or atomic structure of matter. The study of the radio-active substances and of the discharge of electricity through gases has supplied very strong experimental evidence in support of the fundamental ideas of the existing atomic theory. It has also indicated that the atom itself is not the smallest unit of matter, but is a complicated structure made up of a number of smaller bodies.