660 



SCIENCE 



[N. S. Vol. XLIV. No. 1141 



to explain on the assumption that the pos- 

 itive charge is atomic in its dimensions. 



This brings us to the consideration of an 

 atomic structure which is supported by a 

 considerable amount of experimental work. 

 The theory was advanced in 1911 by 

 Rutherford, 7 and was suggested by some 

 experiments of Geiger and Marsden 8 on the 

 scattering of the alpha particles. They ob- 

 served that the alpha particles from a 

 radioactive substance are deflected from 

 their paths on passing through thin films 

 of metal. They found that the amount of 

 scattering varied with the velocity of the 

 alpha particles, with the thickness of the 

 anetal, and with the atomic weight of the 

 metal. It was observed that an occasional 

 particle was deflected through 90° or more 

 and was actually turned back in its course. 

 In order to account for these occasional 

 large deflections, Rutherford assumed that 

 the positively charged alpha particles come 

 into intimate contact with the atoms of the 

 scattering material, and that the deflections 

 are due to the influence of the two electric 

 fields. This would necessitate that the 

 charges be highly concentrated, so he as- 

 sumed that the atom consists of an exceed- 

 ingly small nucleus with a strong positive 

 charge, surrounded by negative electrons 

 distributed throughout the rest of the atom. 

 He then calculated the result of an inti- 

 mate encounter of an alpha particle with 

 the nucleus of an atom, and found that the 

 path of the particle would assume an 

 hyperbolic curve. He calculated the rela- 

 tive number of alpha particles that would 

 be deflected through different angles, and 

 showed that the number of large deflections 

 would be exceedingly small. 



Geiger and Marsden 9 in 1913 made an 

 elaborate series of experiments in order to 



' Ibid., 21, p. 669, 1911. 



s Proc. Boy. Soc, A, 82, p. 495, 1909. 



o Phil. Mag., 5, p. 604, 1913. 



test these theoretical deductions. They ex- 

 perimented with metals of different thick- 

 nesses and different atomic weights, and ob- 

 tained results in accordance with Ruther- 

 ford's calculations. 



According to the theory of Rutherford, 

 when alpha particles pass through hydro- 

 gen gas, an occasional atom of hydrogen 

 should acquire, through an intimate en- 

 counter with an alpha particle, a velocity 

 of 1.6 times, or a range of about 4 times 

 that of the alpha particle. Marsden 10 

 tested this theory experimentally in 1914. 

 He observed that with his apparatus the 

 alpha particles had a range of 20 centi- 

 meters, as determined by their scintillations 

 on a screen of zinc sulphide; and that an 

 occasional hydrogen atom produced scin- 

 tillations as far as 90 centimeters. 



Various lines of investigation support 

 the theory that alpha particles, on passing 

 through matter, occasionally come into inti- 

 mate contact with atomic nuclei, and that 

 the large deflections are due to such en- 

 counters. Wilson's 11 photographs of the 

 actual tracks of alpha particles indicate 

 that such encounters occur. The deflec- 

 tions of the alpha particles obey certain 

 laws which have been worked out by 

 Rutherford on the theory that each atom 

 consists of an exceedingly small nucleus 

 with a positive charge surrounded by nega- 

 tive electrons. The nucleus, in many cases, 

 is probably made up of both positive and 

 negative electrons, the positive charge being 

 always in excess. The algebraic, not the 

 arithmetic, sum of the positive and nega- 

 tive charges in the nucleus represents the 

 nuclear charge, and is always equal to the 

 sum of the charges of the negative electrons 

 surrounding the nucleus. 



Darwin has calculated from the velocity 

 given to the hydrogen atom by the alpha 



io Ibid., 27, p. 824, 1914. 



ii Proc. Boy. Soc, A, 87, p. 277, 1912. 



