1912-13.] The Absorption of Light by Inorganic Salts, No. XI. 157 
ence of energy into the free vibrations by impact ; * while Planck puts it 
down to loss of energy by electromagnetic radiation. 
When we examine these theories and ask what they have done, we find 
that they have explained : 
(1) anomalous dispersion ; 
(2) the Faraday rotation of the plane of polarisation in a magnetic 
field and its connection with the absorption bands ; 
(3) the Zeeman effect observed on the absorption lines of certain 
crystals of the rare earths at low temperatures. 
Of course, to explain these facts it is not absolutely necessary to intro- 
duce electrons and ions, although they are much more satisfactory to the 
mind and give it something concrete to take hold of ; more cautious theorists 
like Yoigt, who discovered (2), have merely supposed that part of the 
electric displacement satisfied the differential equation for forced vibrations. 
But the numerical results obtained by the assumption of electrons and ions 
are very suggestive, and the more general standpoint has found relatively 
few supporters. 
The question naturally arises as to the relation which the electrons 
concerned in the theory of dispersion bear to the atoms and the molecules. 
This question has been tackled by Drude.j- From the constants of the 
dispersion formulae for hydrogen, fluorite, quartz, carbon disulphide, etc., 
he sought to show that in every molecule there was a number of negative 
electrons equal to the sum of the valencies of the atoms in the molecule, 
and also an ion or ions with a total positive charge equal to the sum of the 
negative charges. The periods of the electrons were in the ultra-violet, and 
the periods of the ions were in the infra-red. 
Drude’s results are certainly very interesting. He cannot, however, 
be said to prove them, bi|t only to make them plausible. Indeed, the 
general opinion in this country seems to be that it is asking too much of 
any theory of dispersion to demand information on such points ; that while 
it may give the general features right enough, it must not be pushed into 
too much detail. 
Drude’s views about the absorption bands in the infra-red being caused 
by the vibrations of ions or charged atoms have recently received an 
interesting confirmation from another side. Einstein j assumed that the 
* It is explained in the same way in Schuster’s Optics. 
t “Optische Eigenschaften und Electronentheorie,” Drude’s Ann ., 14 , pp. 677-726, 
pp. 936-961 (1904). 
I A. Einstein, “Eine Beziehung zwischen dem elastischen Verhalten und der 
spezifischen Warme bei festen Korpern mit einatomigen Molekiil,” Ann. d. Phys. (4), 34 , 
p. 170, 1911. 
