[eve] THE CONSTITUTION OF THE ATOM 13 



of a recent alpha particle in an expansion chamber. Some of these 

 photographs showed where a collision had occurred between an alpha 

 particle and one of the heavier molecules of air. It immediately 

 occurred to Sir Ernest Rutherford that a collision between an alpha 

 particle and a lighter atom, such as hydrogen, would result in the 

 nucleus of the latter being projected beyond the known range of the 

 alpha particle. The point was put to the test by Marsden, and a 

 complete justification of Rutherford's nucleus resulted. The hydro- 

 gen nuclei were found to produce scintillations on a zinc sulphide 

 screen at a range about four times as great as that of the alpha particles. 

 Some mathematical investigations by G. C. Darwin indicated that 

 the alpha particle or nucleus of helium, and the hydrogen 

 nucleus must have approached so close that their centres were 

 but 1 • 7 X 10~ 13 cm. apart. This affords further evidence of the extreme 

 minuteness of the nucleus compared with the size of an atom (1CT 8 cm.) 



(9). It may be well to recall at this point an interesting result 

 of Barkla, obtained some years earlier, who showed from the scattering 

 of Rôntgen rays that the number of electrons in the atom must be 

 about £A, where A is the atomic weight. In the case of an uncharged 

 atom, the positive charge on the nucleus must evidently balance 

 the negative charges on the electrons revolving in orbits around that 

 nucleus. 



Thus we can form a clear mental picture of the general character- 

 of the atom. It is a miniature solar system. The sun is represented 

 by the positively charged nucleus, the planets, perhaps confined 

 to one or more definite orbits or rings, by negative electrons revolving 

 rapidly around the nucleus. The gravitational force is replaced by 

 the electrical attraction between the positive nucleus and negative 

 electrons. 



(10). A brilliant young Dane, Bohr, has gone a step farther 

 and suggested the structure of an atom capable of explaining the 

 series of spectral lines. His work is remarkable as leading to ex- 

 cellent numerical verification. He assumes the Rutherfordian nucleus 

 of electronic charge about half the atomic weight; he also assumes 

 that for every revolving electron in every atom the angular momentum 

 is constant. To be concise he supposes that for each electron, 

 mass X velocity X radius = Planck's constant /2 ~. 



He further supposes that in a steady stationary orbit even a 

 single electron does not radiate away energy. This is entirely con- 

 trary to classical electrodynamics. Furthermore he imagines that in 

 passing from one state of stationary orbit to the next possible, there 

 is homogeneous radiation of amount hn, where n is the frequency. 



