Dispersion of Metals. 671 



continuous absorption throughout the spectrum. That the 

 character 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 connexion, 

 it is a natural inference 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 tbe 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 band at the upper end of the spectrum plainly 

 discernible for silver, and indicated for copper and gold, 

 would seem to make certain. Further evidence in confirma- 

 tion 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 

 electrons should absorb less of the incident energy at higher 

 frequencies. 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 w 7 ould be capable of 

 elucidation. At present, however, and certainly as far as 

 the experiments under discussion go, there seems to be no 

 light on this question. If the fact of decreasing electronic 

 absorption with increasing frequency be granted, nevertheless^ 

 then the above hypothesis appears to be sufficient to reconcile 

 the experimental data *. 



* It may be possible that the increase in r is to be attributed to a 

 transition layer or region of interpenetration of air and metal. This 

 might naturally be supposed to be relatively poorer in free electrons 

 than the main body of the metal. Then, if at the longer wave-lengths 

 the radiation did not penetrate this layer, while at the shorter it did, 



