MASTER KEY OF SCIENCE RtJSSELL 137 



give widely separated bands. From these, new isotopes of oxygen, 

 nitrogen, and carbon have been discovered, and the ratio of the 

 masses of the different atoms determined with extreme precision. In 

 atomic spectra, the isotope effect is extremely small, except for hydro- 

 gen—where it has recently permitted the identification of an isotope 

 of double weight. 



Fine structure in the lines of heavier atoms arises partly from the 

 presence of isotopes and partly from some sort of " spin " within 

 the atomic nucleus; and its study affords a promising approach to 

 the problem of nuclear structure. 



While all this was going on, X rays were also found to contain 

 monochromatic radiations, observable by using the atoms in a crystal 

 as a diffraction grating. These spectra have given us information 

 about the interior of atoms, comparable with that which optical spec- 

 tra furnish concerning the exterior. They are much simpler than 

 the latter, and now furnish the chemist with his most delicate test 

 for the detection of new elements. Incidentally, they make it certain 

 that except for the few well-recognized gaps, no elements lighter 

 than uranium remain to be discovered. 



Working in the opposite direction. X-ray spectroscopy opens the 

 door to another untrodden realm — the exact study of the arrange- 

 ment of atoms in crystals, which can now be specified in minute 

 detail. 



All through these triumphs ran a discordant note. Certain numer- 

 ical relations — notably in the Zeeman effect — though exact, differed 

 systematically from those predicted by the orbit theory, and every 

 calculation based on the relative positions of electrons in these orbits 

 led to a wrong answer. This discrepancy has vanished since the 

 orbital picture of the atom was replaced by the difficultly visualizable 

 wave-mechanics or the wholly unpicturable matrix theory. When a 

 modern lecturer tries to draw an atom on the blackboard, he uses not 

 chalk, but an eraser, and constructs a smudge illustrating the relative 

 probability of finding a unit charge in different regions. But as a 

 means of calculation — interpreting and, on occasion, predicting the 

 results of precise observation — the new theory advances from con- 

 quest to conquest. 



The ramifications of these new ideas throughout the range of mo- 

 lecular and atomic physics are too numerous to mention. To take 

 but one instance at random, the magnetic susceptibilities of solutions 

 of salts of the rare earths may be fully explained by the theory of 

 spectral structure — even though the spectra of the trebly ionized 

 atoms (upon which these depend) have not yet been observed. 



There is probably no field in which the new spectroscopy has been 

 of more aid than in astrophysics. The recognition that Lockyer's 



