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SCIENCE 



[N. S. Vol. XL. No. 1021 



has come to stay, for the applications of the 

 quantum hypothesis have already achieved 

 a great and unexpected measure of success. 

 In the meantime it is necessary to proceed 

 with caution, checking every theory by 

 experiment, for there is no other criterion 

 to guide the investigator, whether to hold 

 to the old or try the new. 



7. The first steps towards the idea of the 

 modern or Eutherfordian atom rest on an 

 experimental basis, and are not, therefore, 

 open to suspicion. 



Rutherford and Geiger found that when 

 the alpha particles from a radiant, such as 

 radium or polonium, met a thin gold leaf, 

 the bulk of the alpha particles passed 

 through with slight deflection, but about 

 one in eight thousand bounced back, or re-' 

 turned towards the side of their source. 

 Both large and small deviations of the 

 alpha particles in passing through matter 

 were satisfactorily explained by ordinary 

 or Newtonian djmamics, with the law of 

 repulsion inversely as the square of the 

 distance between similar electric charges. 

 One charged particle was the alpha par- 

 ticle with a positive charge twice as large, 

 numerically, as that of an electron. The 

 other charged particle was the nucleus of 

 the atom of gold, and the magnitude of this 

 charge was about ^A where A is the atomic 

 weight of gold. This view was subjected to 

 a searching series of experimental tests and 

 emerged triumphant. 



8. About this time 0. T. R. Wilson skil- 

 fully obtained photographs of the mist- 

 ladened, charged air molecules, marking 

 the track of a recent alpha particle, in an 

 expansion chamber. Some of these photo- 

 graphs showed where a collision had oc- 

 curred between the alpha particle and one 

 of the heavier molecules of air. It imme- 

 diately 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 justi- 

 fication of Rutherford's nucleus resulted. 

 The hydrogen nuclei were found to pro- 

 duce scintillations on a zinc sulphide 

 screen at a range about four times as great 

 as that of the alpha particles. Some mathe- 

 matical 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 centers 

 were but 1.7 X 10"" cm. apart. This 

 affords further evidence of the extreme 

 minuteness of the nucleus compared with 

 the size of an atom (10"^ em.). 



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 Rontgen 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 re- 

 placed by the positively charged nucleus. 

 The planets, perhaps confined to one or 

 more definite orbits or rings, are replaced 

 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 struc- 

 ture of an atom capable of explaining the 

 series of spectral lines. His work is re- 

 markable as leading to excellent numerical 

 verification. He assumes the Rutherfordian 

 nucleus of electronic charge about half the 



