ABSORPTION AND EMISSION CENTERS. 9 



and shows that the absorber of all the shorter waves of the spectrum is the 

 negative electron. 



Throughout this part of the present monograph the absorbers are con- 

 sidered as negative electrons. These electrons have certain free periods cor- 

 responding to the bands of selective absorption. These free periods are greatly 

 modified by the presence of certain chemical radicals, and seem to be elec- 

 trons that are situated either in the outer parts of the atom or between two 

 or more atoms. Stark and others call these the valency electrons, and con- 

 sider that chemical valency is due to them. Chemical bonds will then be 

 closely associated with the electric fields of these electrons. While the theory 

 in its present state is confronted with many difficulties, yet it seems a step 

 towards the explanation of the more or less vague chemical bond. As an aid 

 to our imagination, we shall consider atoms or ions as large spherical regions 

 throughout which a positive charge is uniformly distributed. These regions 

 are sometimes spoken of as "spheres of influence." Two atoms collide when 

 their "spheres of influence" touch. Groups of atoms composing ions, radicals, 

 or molecules will have "spheres of influence." No ion can penetrate the 

 sphere of influence of another atom or molecule. On the other hand, the elec- 

 trons are very small and bear much the same relations in size to the atom 

 that the sun bears to the solar system. The electric fields of the electrons, 

 however, occupy quite large volumes, although the energy of this field is for 

 the most part situated in a very small space. Electrons can, therefore, move 

 through ions and atoms if they have sufficient velocity. In most organic com- 

 pounds it is considered that the valency electrons move in the interatomic 

 spaces with considerable ease. In the metals a large number of the electrons 

 are free. In organic compounds that are transparent to certain wave-lengths, 

 the electrons in general will be held within certain regions by forces that are 

 supposed to be elastic in their nature. 



A considerable amount of work has been done by Koenigsberger and 

 Kichling 1 on the determination of the coefficients of absorption of organic 

 coloring matters, compounds of the metalloids, minerals, etc. Selective 

 absorption is classified under two heads. In the first class the absorbing and 

 reflecting powers are closely connected, and the absorption is probably due to 

 electrons or ions vibrating in the chemical molecule; in the second class the 

 absorption has no effect on the reflecting power, and the absorbers in this case 

 are rare and are not connected with the molecular structure. 



Letting p be the number of electrons per molecule, it is found that absorp- 

 tion curves give the value of p e/m better than the dispersion curves. For 

 organic coloring matters p e/m = 1.3(H)) 7 . For a temperature of absolute 

 zero, they consider that p e/m = 1.78(H)) 7 . This quantity decreases as the 

 temperature rises to the probable limit l / 2 1.78(10) 7 . For the metalloids there 

 is a single vibrating electron for the bromine family of elements, two for the 

 selenium family, and three for the phosphorus family. 



For a large number of substances, the absorption curve is displaced towards 

 the red when the temperature is raised. It widens and becomes flatter at the 

 same time. From this it seems that the quasi-elastic force that acts on the 

 electron decreases with rise in temperature. 



1 Ann. .1. Phys., 28, 889 (1909); 32, 843 (1910); PHys. Zeit., 12, 1 (1911). 



