CONTEMPORARY ADVANCES IN PHYSICS 85 



the line marked 2564, which is a faint line of the primary light scattered 

 without change in frequency. This diminution in the shift signifies 

 that the natural frequencies of the lattice-vibrations are declining, 

 which is to be expected from the expansion and relaxation of the 

 crystal which the rise of temperature brings about. 



There arises now an interesting question. X-ray analysis reveals 

 that there are certain chemical compounds, organic chiefly, of which 

 the molecules retain their identity when crystallization occurs; they 

 set themselves side by side in a regular lattice, but the arrangement 

 of the atoms in each of them is not greatly altered. At the other 

 extreme, there are compounds of which the molecules disintegrate 

 completely when solidification takes place, and the atoms arrange 

 themselves without any reminiscence of their earlier relations ; a familiar 

 instance is sodium chloride, in the crystal of which every atom of 

 either kind, Na or CI, is surrounded by six of the other kind all equally 

 distant from it. Intermediate cases occur, as for instance that of 

 CaCOs, where each atom-group or "radical" CO3 retains its identity 

 but not its coupling to one single Ca atom. Now when molecules or 

 radicals survive within the crystal, oscillations of atoms inside these 

 atom-groups are probably not different in character from the oscilla- 

 tions which occur within the same molecules when they are wandering 

 freely in a liquid or a gas. But in the case of a crystal like sodium 

 chloride, the oscillations must be controlled by the forces which hold 

 the atoms of the crystal together; they are truly lattice-vibrations. 

 Perhaps the difTerence between the two is not really profound; but 

 it will be interesting to find out whether quanta may or may not 

 transfer energy with equal ease to vibrations of either type, i.e. whether 

 in the two cases the shifted lines are comparably bright. According 

 to Carrelli, Pringsheim and Rosen, all of the shifted lines thus far 

 observed with solids, except probably those obtained with quartz, 

 correspond to vibrations within molecules or radicals which remain 

 intact in the crystal. 



The Compton Effect 



The Compton efTect, in the restricted sense — the sense in which I 



shall use the term — is simply the scattering ensuing on collisions of 



corpuscles of light with free electrons. So much has been written 



about the efTect ^^ that it is scarcely necessary for me to do more in 



this place than mention the laws of these collisions. The energy which 



the quantum loses is converted into kinetic energy of translatory 



1" Cf. for instance this Journal, April, 1925; "Introduction to Contemporary 

 Physics," pp. 146-160; H. Kallmann, H. Mark, "Ergebnisse der exakten Natur- 

 wissenschaften," 5 (1926). 



