11114] on X-Rays and Crystalline Structure 2o5 



the magnitude of fche nucleus of any element as being simply propor- 

 tional to the number indicating the place of the element in the 

 periodic table, hydrogen having a nuclear change of one unit, helium 

 two, and so on. The atomic weights of the successive elements do 

 not increase in an orderly way ; they mount by steps of about two, 

 l)ut not very regularly, and sometimes they seem absolutely to get 

 into the wrong order. For example, nickel has an atomic weight of 

 58 • 7, whereas certain chemical properties and still more its behaviour 

 in experiments on radio-activity indicate that it should lie between 

 cobalt (59) and copper (63-6). But the wave-lengths, which are now 

 our means of comparison, diminish with absolute steadiness in the 

 order cobalt, nickel, copper. Plainly, the atomic number is a more 

 fundamental index of quality than the atomic weight. 



It is very interesting to find, in the series arranged in this way, four, 

 and only four, gaps which remain to be filled by elements yet un- 

 discovered. 



Let us now glance at another and most important side of the 

 recent work, the determination of crystalline structure. We have 

 already referred to the case of the rocksalt series, but we may look at 

 it a little more closely in order to show the procedure of crystal 

 analysis. 



The reflection of a pencil of homogeneous rays by a set of crystal- 

 line planes occurs, as already said, at a series of angles regularly 

 increasing ; giving, as we say, spectra of the first, second, third 

 orders, and so on. When the planes are all exactly alike and equally 

 spaced the intensities of the spectra decrease rapidly as we proceed to 

 higher orders, according to a law not yet fully explained. This is, for 

 example, the case with the three most important sets of planes of 

 sylvine, those perpendicular to the cube edge, the face diagonal, and 

 the cube diagonal respectively. An examination of the arrangement 

 of the atoms in the simple cubical array of sylvine shows that for all 

 these sets the planes are evenly spaced and similar to each other. 

 It is to be remembered that the potassium atom and the chlorine 

 atom are so nearly equal in weight that they may be considered 

 effectively equal. In the case of rocksalt the same may be said of the 

 first two sets of planes, but not of the third. The planes perpen- 

 dicular to the cube diagonal are all equally spaced, but they are not 

 all of equal effect. They contain alternately, -chlorine atoms (atomic 

 weight 35 • 5) only and sodium atoms (atomic weight 23) only. The 

 effect of this irregularity on tlie intensities of the spectra of different 

 orders is to enhance the second, fourth, and so on in comparison with 

 the first, third, and fifth. The analogous effect in the case of light is 

 given by a grating in which the lines are alternately light and heavy. 

 A grating specially ruled for us at the National Physical Laboratory 

 shows this effect very well. This difference between rocksalt and 

 sylvine and its explanation in this way constituted an important link 

 in W. Lawrence Bragg's argument as to their structure. 



