L. P. Wheeler — Dispersion of Metals. 505 



taking up of the incident energy by the free electrons, and the 

 second the expenditure of the incident energy in forcing vibra- 

 tions in the relatively fixed ions. The absorption due to the 

 second cause must be supposed to vary with the wave length in 

 such a way as to be relatively much greater in the neighbor- 

 hood of those wave lengths corresponding to the natural free 

 periods of the ions. In other words, the ions should produce a 

 " band " type of absorption, the breadth of the bands being 

 proportioned to the range of frequencies to which they are 

 capable of responding. On the other hand, the electrons being 

 free and of small mass should take up energy (if at all) from a 

 much wider range of frequencies, and hence cause a more con- 

 tinuous absorption throughout the spectrum. That the char- 

 acter of the absorption of metals is, for the longer wave lengths 

 at least, of this second type is obvious from an inspection of 

 the curves shown in the several figures. Moreover, as the 

 presence of free electrons is theoretically the distinguishing 

 characteristic of metals in this connection, it is a natural infer- 

 ence that the major part of the absorption of light by metals is 

 due in general to the electrons. 



If this is so, then it would be possible to imagine the total 

 absorption due to the two causes to decrease while that due to 

 the second was increasing ; and even that there should be a 

 minimum of the total absorption coincident with or nearly 

 coincident with a maximum of the ionic absorption. Then, if 

 in such a resonance region the disturbance within the ions is 

 great enough to set free some electrons, the increase in r would 

 be accounted for. Further, these same considerations would 

 indicate that as the part due to the electrons becomes less, the 

 resultant absorption should change its type, and that it should 

 take on more of the character of the absorption in transparent 

 bodies. That such a change really does occur, the absorption 

 bands at the upper end of the spectrum plainly discernable for 

 silver and indicated for copper and gold would seem to make 

 certain. Further evidence in confirmation of such a change in 

 the type of absorption is offered by the great relative increase 

 in the values of n in the cases of silver, copper, and gold at or 

 after the minimum point of the absorption curve. 



The above hypothesis offers no explanation of why the elec- 

 trons should absorb less of the incident energy at higher fre- 

 quencies. If it were possible to form a sound dynamical 

 notion as to why the electron with its minute dimensions and 

 mass and simple structure should take up any energy from 

 waves which at the shortest used in these measurements are 

 still more than ten million times the electronic diameter, then 

 perhaps the mechanism of the indicated decrease in absorption 

 with increasing frequency would be capable of elucidation. 



