ERNEST RUTHERFORD NOBEL LECTURE December 11, 1908. THE CHEMICAL NATURE OF THE ALPHA PARTICLES FROM RADIOACTIVE SUBSTANCES The study of the properties of the α-rays has played a notable part in the development of radioactivity and has been instrumental in bringing to light a number of facts and relationships of the first importance. With increase of experimental knowledge there has been a growing recognition that a large part of radioactive phenomena is intimately connected with the expulsion of the α-particles. In this lecture an attempt will be made to give a brief historical account of the development of our knowledge of the α-rays and to trace the long and arduous path trodden by the experimenter in the attempts to solve the difficult question of the chemical nature of the α-particles. α-rays were first observed in 1899 as a special type of radiation and during the last six years there has been a persistent attack on this great problem, which has finally yielded to the assault when the resources of the attack seemed almost exhausted. Shortly after his discovery of the radiating power of uranium by the photographic method, Becquerel showed that the radiation from uranium like the Röntgen-rays possessed the property of discharging an electrified body. In a detailed investigation of this property, I examined the effect on the rate of discharge by placing successive layers of thin aluminium foil over the surface of a layer of uranium oxide and was led to the conclusion that two types of radiation of very different penetrating power were present. The conclusions at that period were summed up as follows: