2 A STUDY OF THE ABSORPTION SPECTRA. 



wave. If the surface is rough, a great number of so-called waves will be 

 reflected. The remaining part of the disturbance will advance through 

 the body. As no body is a perfect reflector or absolutely transparent, it 

 follows that part of the energy of the light-wave remains with the body. 

 This phenomenon is known as the absorption of light. We also know of 

 many conditions of matter in which light is emitted. The object of the 

 study of emission and absorption of light is to gain some knowledge of the 

 mechanism of matter by which it is enabled to produce or absorb electro- 

 magnetic waves. The expression of the properties of different kinds of 

 matter by different values of the dielectric constant (/c), the conductivity 

 (a) or the magnetic permeability (/x) has not been found to be satisfactory. 



The electromagnetic mechanism which at present is considered as the 

 basis of the theories of radiation and absorption is the electron. The 

 charge which it carries has been found to be the atomic unit of electricity. 

 Experimental results in electricity can be explained on this basis. The 

 electron is found in the vacuum discharge-tube, in the radiations from 

 radioactive matter, in arcs, in sparks, in secondary radiations. They are 

 present in all bodies. By the distribution and motions of these electrons men 

 of science to-day attempt to explain all electrical and optical phenomena. 

 Some electrons in a conducting body are in a free state, so that they can 

 obey an electric force. Richardson and Brown 1 have shown that ions 

 emitted by hot platinum (and approximately so for other metals) are 

 kinetically identical with the molecules of a gas, of equal molecular weight, 

 at the temperature of the metal. This holds for the mode of distribution 

 of velocity as well as its average value, and shows that the free electrons 

 inside the metal have the same amount and mode of distribution of velocity 

 and kinetic energy as the molecules of a gas of equal molecular weight at 

 the temperature of the metal. 



In the case of a nonconducting substance the electrons are considered 

 as bound to certain positions of equilibrium. In a conductor in an electric 

 field there is an excess of electrons at one end. In a dielectric, as soon as 

 an electron is displaced from a position of equilibrium, a new (elastic) 

 force is brought into play which pulls the electron back to its original posi- 

 tion. The motion of electrons in nonconducting bodies, together with the 

 change of dielectric displacement of the ether itself, makes up Maxwell's 

 displacement-current. Under the influence of the elastic forces the elec- 

 trons can vibrate about their positions of equilibrium and may thus become 

 the centers of electromagnetic waves. In this way may be explained the 

 emission of light and heat. Absorption results when the electrons are set 

 into vibration by a beam of light, and part of the vibrating energy of the 

 electron is transformed into heat energy. 



As to the nature of the electron very little is known. On the other 

 hand, the mathematical electron is much better known. As the recent 

 experiments by Bucherer 2 on the value of e/ra agree with values calculated 

 by Lorentz, use will be made here of his conception of the electron. To 

 each electron is ascribed certain definite dimensions. The ether is assumed 



1 Phil. Mag., 16, 353 and 740 (1908). 2 Phys. Zeit., 9, 755 (1908). 



