Optical and Electrical Qualities of Metals. 157 
and would then become inappreciable, or at any rate so small 
that it could not be discharged from the probable effects of 
selective absorption. The effect of these greater masses is 
not denied, but their existence would not sensibly affect our 
present results, which apply only to the electron of small 
mass carrying a negative charge. Drude, in his discussion of 
optical constants in connexion with his electron theory of metals, 
assumed the existence of two kinds of conducting electrons, 
and could by means of them account separately for the observed 
values for » and for «. 
My purpose is a more limited one, but the results obtained 
seem to me to be more definite on account of the small 
number of assumptions which are made. 
Looking at the results of these calculations, the simplest 
explanation of metallic conduction would appear to be that 
each atom has one, or possibly two or three, negative elec- 
trons which are easily detached, and follow freely the electric 
force, even for such rapid oscillations as those of light. 
Drude assumes that they are always detached from the atom 
and behave very much as the ions in electrolyies are supposed 
to behave. He has worked out his idea in two important 
papers *, and given it substantial support in a variety of 
directions. 


XVI. On some Relations between the Optical and the Elec- 
trical Qualities of Metals. By Prof. E. Hagen and 
Prof. H. Rupensf. 
AXWELL’S electromagnetic theory of light—which in 
its original form does not consider the molecules and 
their vibratory periods, but simply expresses the optical 
property of a single wave-length—demands the existence of 
analogous relations between the transparencies of metals and 
their electric conductivities. These relations have often been 
examined, without being confirmed in any way}. The theory 
did not seem to hold good, either with regard to the absolute 
amount of transparency, or with regard to the order in 
which the metals can be arranged according to their trans- 
parency, and which ought to be the order of their electrical 
resistances. 
* Wied. Ann. xxxix. p. 537, xlii. p. 189. 
¥ Cf. Sitzungsberichte der K. Akademie der Wissensch. in Berlin, 
p. 269 & p. 410 (1903), and Ann. d. Physik, xi. p. 873 (1903). Authors’ 
translation communicated by Prof. C. Vernon Boys, F.R.S. 
t C&. W. Wien, Wied. Ann. xxxv. p. 48 (1888) ; and E. Cohn, Wied. 
Ann, xly. p. 55 (1892), and others. 
