On a diffuse reflection of the alpha-particles. 1909

London: Harrison & Sons, 1909. First edition, the extremely rare offprint, of the famous Geiger-Marsden experiment (or gold foil experiment) which demonstrated for the first time the existence of the atomic nucleus, leading to the downfall of Thomson’s plum-pudding model of the atom, and the development of the Rutherford (or planetary) model (1911), and then to Bohr’s quantum theory of atomic structure (1913). No copies of this offprint in auction records; no copy located in institutional collections worldwide.<br/><br/>“One of the most important experiments in physics took place in 1909 when Hans Geiger (1882-1945) and the undergraduate student Ernest Marsden (1889-1979), under the direction of Rutherford, sent alpha particles towards a very thin film of gold, and discovered that the majority of them passed through the foil without hitting anything. Only a tiny number of particles were scattered back (towards the source) after hitting the nucleus of a gold atom. The results of the experiment were analyzed by Rutherford (the experimental results were first described by Geiger and Marsden in Proc. Roy. Soc., vol. 82, p. 495, 1909), and led to several far-reaching conclusions. The first was that most of the atom is empty! The nucleus occupies only the 10^-15th part of the volume of the atom (a radius 105 times bigger than the newly found radius of the nucleus). Thus, the orbits of the outer-most electrons (which define the radius of the atom) are far away from the nucleus. The second conclusion was that the force acting between the positive charge in the nucleus and charged scattered projectiles obeys the Coulomb law” (Shaviv, <i>The Life of Stars</i>, Springer 2009). <br/><br/>“In the work of Rutherford and Geiger on counting alpha particles by the electric method, carried out in 1908, some of the difficulties that had to be overcome were due to the scattering of alpha rays in passing through matter. Geiger made a special study of the scattering for small angles of deflection, and in 1909 Rutherford suggested to one of his research students, E. Marsden, an examination of the possibility of scattering through large angles. As a result of this suggestion, experiments were carried out by Geiger and Marsden, which showed [in the present paper] that alpha particles fired at a thin plate of matter can be scattered inside the material to such an extent that some of them emerge again on the side of the plate at which they entered: and calculation showed that some of the alpha particles must have been deflected at single encounters through angles greater than a right angle” (Whittaker, <i>A History of Theories of Aether and Electricity</i>, Vol. 2, p. 20). <br/><br/>Rutherford later explained his reaction to this startling discovery: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration I realized that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greatest part of the mass of the atom was concentrated in a minute nucleus.” <br/><br/>At the time Geiger & Marsden performed their experiment, most physicist accepted the ‘plum pudding model’ of the atom put forward in 1904 by J. J. Thomson, according to which the electrons in an atom were floating in a sea of positive charge. The results of Geiger & Marsden’s experiment disproved this model, since if the positive charge were spread out only small deflections of the alpha-particles would occur. To explain the results, Rutherford proposed that an atom consists of a small positively-charged ‘nucleus’ surrounded by electrons. This was published two years later (‘The Scattering of α and β Particles by Matter and the Structure of the Atom,’ <i>Philosophical Magazine</i>, Series 6, Vol. 21 (1911), 669-88). Rutherford’s description of the atom laid the foundation for all future atomic models and the development of nuclear physics.<br/></br>DSB, V, 331; XII, 31; Brandt, <i>The Harvest of a Century</i>, pp. 66-69.

Offprint from Proceedings of the Royal Society, Series A, Vol. 82, No. 557, July 31, 1909. 8vo (252 x 177 mm), pp. 495-500. Original printed wrappers (two punch-holes to inner margin filled, library stamp to front wrapper).