204 Professor William H. Bragg [June 5, 



atoms in rocksalt is 2*^1 x 10"^ cm., which distance is also the 

 spacing of the planes parallel to a cube face. From a knowledge 

 of this quantity the length of any X-ray wave can be calculated at 

 once as soon as the angle of its reflection by the cube face has been 

 measured. In other words, the spectrometer has now become a means 

 of measuring the length of waves of any X-radiation, and the actual 

 spacings of the atoms of any crystal. 



From this point the work branches out in several directions. It 

 will not be possible to give more than one or two illustrations of the 

 progress along each branch. 



Let us first take up the most interesting and important question 

 of the " characteristic " X-rays. It is known that every substance 

 when bombarded by electrons of sufficiently high velocity emits 

 X-rays of a quality characteristic of the substance. The interest of 

 this comparison lies in the fact that it displays the most fundamental 

 properties of the atom. The rays which each atom emits are charac- 

 teristic of its very innermost structure. The physical conditions of 

 the atoms of a substance and their chemical associations are largely 

 matters of the exterior : but the X-rays come from the interior of 

 the atoms and give us information of an intimate kind. What we 

 find is marked by all the simplicity we should expect to be associated 

 with something so fundamental. 



All the substances of atomic weight between about 30 and 120 

 give two strongly defined " lines " ; that is to say, there are found 

 among the general heterogeneous radiation two intense almost homo- 

 geneous sets of waves. For instance, rhodium gives two pencils of 

 wave-lengths, approximately equal to 0'61 x 10"^ cm. and 

 0-54 X 10-^ cm. respectively. More exactly the former of these 

 is a close doublet having wave-lengths 0'619 X 10~^ and 

 • 614 X 10-^. The wave-lengths of palladium are nearly * 58 x 10"^ 

 and 0-51 xl0-« ; nickel l-'66 x 10-» and 1-50 xlO-^. Lately 

 Moseley has made a comparative study of the spectra of the great 

 majority of the known elements, and has shown that the two-line 

 spectrum is characteristic of all the substances whose atomic weights 

 rauge from that of aluminium, 27, to that of silver, 108. These X-rays 

 constitute, there is no doubt whatever, the characteristic rays which 

 Barkla long ago showed to be emitted by this series of substances. 



Now comes a very interesting point. When Moseley sets the in- 

 creasing atomic weights against the corresponding decreasing wave- 

 lengths, the changes do not run exactly parallel with each other. But 

 if the wave-lengths are compared with a series of natural numbers 

 everything runs smoothly. In fact it is obvious that the steady de- 

 crease in the wave-length as we pass from atom to atom of the series 

 in the periodic table implies that some fundamental element of atomic 

 structure is altering by equal steps. There is excellent reason to 

 l)elieve that the change consists in successive additions of the unit 

 electric charge to the nucleus of the atom. We are led to think of 



