July 24, 1914] 



SCIENCE 



119 



atomic weight; lie assumes tliat for every 

 revolving electron in every atom the 

 angular momentum is constant. To be con- 

 cise, he supposes that for each electron 

 mass X velocity X radius = Planck's con- 

 stant /27r. 



He further supposes that in a steady 

 stationary orbit even a single electron does 

 not radiate away energy. This is entirely 

 contrary to classical electrodynamics. Fur- 

 thermore 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. This 

 is of course Planck's assumption, and it is 

 certainly unexplained, and probably not 

 in accord with Hamilton's equations as 

 deduced from Newton 's laws. Nevertheless, 

 any day we may learn why energy is 

 emitted per saltum, and this mystery will 

 vanish. 



Now if you permit these somewhat arbi- 

 trary assumptions to Bohr, he can and does 

 deduce, at least for the lighter atoms such 

 as hydrogen and helium, the Rydberg 

 formula for the spectral series. He finds: 



__277W / 1 _ 1\ 



where n is the frequency; m, e, mass and 

 charge of an electron; h is Planck's con- 

 stant ; a, i, are integers. The quantity be- 

 fore the bracket should equal the Rydberg 

 number N^, of observed value 3.29 X 10^°. 

 Bohr's calculated value is 3.26 X 10^°> 

 showing a most satisfactory agreement. 



Bohr endeavors to account for the man- 

 ner in which two hydrogen atoms form a 

 molecule. Each atom has a nucleus of 

 positive charge and a simple electron re- 

 volving around it. Their charges are equal 

 and opposite. The nuclei of two such 

 atoms repel each other. The revolving 

 electrons of two atoms close together, if 

 rotating in the same direction, constitute 

 two parallel currents of electricity, and 



these attract one another and arrive in the 

 same plane. It is easy to make a model on 

 a whirling table with the nuclei on an up- 

 right rod, the electrons revolving like the 

 governor balls of an engine. Bohr has gone 

 further, and conceived a similar model of 

 a water molecule with the two nuclei of 

 hydrogen and one nucleus of oxygen in a 

 straight line, with 10 electrons revolving 

 in their zones around them. No doubt 

 these suggestive schemes are somewhat 

 speculative, but it is refreshing to find a 

 first approximation to a djmamical scheme 

 replacing the old unsatisfactory electro- 

 static atoms, which probably did not ap- 

 proximate to the truth. Some of the 

 formidable organic molecules must have a 

 complexity which it may take generations 

 of physicists to unravel. 



11. One of the triumphs of mathematical 

 physics was the forecast of Laue that crys- 

 tal bodies have their atoms so distributed 

 that Rontgen rays must be diffracted by 

 these atoms in the same manner that closely 

 ruled crossed lines diffract visible light. 

 This forecast and its rapid verification, en- 

 able the two Braggs, father and son, to 

 measure with accuracy the wave-lengths of 

 Rontgen rays. "While the waves of visible 

 light are of the order 10"° cm., those of 

 Rontgen rays are of the order 10"' cm., 

 about one thousandth of the former. The 

 electromagnetic theory recognizes no in- 

 trinsic difference between the great waves 

 of wireless telegraphy, several kilometers in 

 length (10" cm.), short electric waves, long 

 heat waves, visible light (10"° cm.), ultra- 

 violet waves, and Rontgen rays (10"^ cm.). 



The method of reflecting Rontgen rays 

 from a rock-salt or another crystal has been 

 applied by Moseley with marked success 

 to the determination of the nucleus charges 

 of the atoms of most of the elements. He 

 bombarded the elements one after the 

 other, by electrons as cathode rays, reflected 



