The New Zealand-born physicist Ernest Rutherford was one of the dominant figures of early modern physics, perhaps one of the greatest experimental physicists of all time. Born in Spring Grove (later Brightwater), the fourth of 12 children, Rutherford won scholarships to Nelson College and Canterbury College, Christchurch. His first research projects involved the magnetization of iron by high-frequency discharges and magnetic viscosity. In 1895 Rutherford was admitted to the Cavendish Laboratory and Trinity College, Cambridge University, and in 1898 he became professor of physics at McGill University in Canada. From McGill he went back to England to Manchester University and then to the Cavendish Laboratory again. As director of the Cavendish Laboratory, he attracted some of the best young physicists in the world. Rutherford's achievements are many. He was the first to explain that radioactivity is produced by the disintegration of atoms, distinguishing among three types of radioactive emission---alpha rays, beta rays, and gamma rays. For this achievement, he was awarded the Nobel Prize in chemistry in 1908. In 1911 Rutherford conceived a new model that clarified the structure of the atom. Rutherford's nuclear model predicted that almost all of the mass of an atom is concentrated in a very small central region (the nucleus), while most of the remaining area consists of empty space. Rutherford also did extensive work with the natural and artificial transmutation of radioactive elements, and was the first to suggest the presence of a neutral particle in all atomic nuclei (although neutrons were not isolated until 1932). When Rutherford began exploring the phenomenon of radioactivity, little more was known than that it was a phenomenon that characterized uranium and other elements. Working with the English chemist Frederick Soddy at McGill University, he explained radioactivity as a phenomenon caused by the breakdown of atoms in a radioactive element to produce a new element. The two men discovered that the intensity of the radioactivity decreases at a rate determined by the element's half-life. The notion that atoms could change their identity was a revolutionary idea, yet Rutherford's explanation was so satisfactory that it found immediate acceptance in the scientific community. Rutherford utilized the notion of natural transmutation of elements to calculate the ages of mineral samples, arriving at figures greater than a billion years. This was the first proof of the great age of the Earth's rocks, and the process of radioactive dating has since been developed and applied to fossils and archaeological remains as well. With Hans Geiger, Rutherford developed a particle counter to measure the radioactivity released by elements. Perhaps his greatest discovery concerned the structure of the atom. Before Rutherford's work, an atom was pictured as a sphere of positive charge occupying the whole volume of the atom. Negatively charged electrons were thought to be embedded in this space, rather like raisins in a raisin cake. This model had to be abandoned when a student of Geiger and Rutherford, Ernest Marsden, made a series of measurements of the unexpected scattering of alpha particles by thin metal foils. It was observed that a few of the particles bombarding the foil were reflected back. As Rutherford remarked, "It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." Rutherford was forced to assume that most of the atom's mass was concentrated in a small space, or nucleus. Gradually, this new model of the atom came to be the accepted one among scientists.
