96 INFRA-RED REFLECTION SPECTRA. 



The present list can not, of course, be of practical use, since the sur- 

 faces tarnish, but from a theoretical standpoint they are of considerable 

 importance. For example, Hagen and Rubens established relations 

 between the reflecting power and the electrical conductivity of the 

 metals studied. One would, therefore, expect similar relations for 

 closely related metals in the Mendeleef's series. 



For example, one would expect the reflecting power of cobalt to be 

 of the same order as that of nickel, and a similar relation between zinc, 

 and copper, and palladium and platinum. From the present examina- 

 tion it will be noticed that such a close parallelism exists in all cases 

 where the actual condition of the reflecting surface, i. e., its polish, is 

 negligible. Unfortunately in the present list only zinc and cobalt take 

 a high polish which is quite permanent. Cadmium also takes a high 

 polish, but tarnishes in a day or two. Tin can not be given a high 

 polish ; palladium is of a similar nature, while aluminum always retains 

 a hazy white surface. As a result, in the shorter wave-lengths the 

 reflecting power is lower than normal, and rises steadily to 8 or 10 /*, 

 where it assumes a constant value which can no doubt be interpreted 

 as real. 



The specimens examined were about 3 by 4 cm. area. They were 

 ground plane, then polished with Vienna lime and stearin oil. The 

 silver mirror was finally prepared by "buffing," and had a fine surface. 



An attempt was made to use silver-on-glass mirrors, but even those 

 that transmitted only blue light were found to differ as much as 2 per 

 cent in reflecting power, while the best silver-on-glass mirror reflected 

 about 0.5 per cent less than the one of pure silver at 5 ju, to io/t. A 

 mirror of pure silver was therefore used as a standard of reference. It 

 consisted of a thick (0.5 mm.) sheet of the pure metal soldered on a 

 heavy plate of brass. 



The method of observation consisted in placing the standard silver 

 mirror and the comparison mirror upon the carrier before the spectro- 

 meter slit, as in the preceding work, and obtaining the ratio of the 

 deflections. This gives the reflecting power relative to silver, and is 

 slightly higher than the absolute reflecting power, since silver is not a 

 perfect reflector. The absolute reflecting power of the metals, given 

 in Table I, were found by multiplying the relative values by the reflect- 

 ing power of massive silver, given in the first column of the same table. 



The reflecting power of Ni and Pt are quoted from Hagen and 

 Rubens (loc. cit.) to show their close relations with Co, Pd, and Ir, 

 respectively. The angle of incidence was about 25, and was permis- 

 sible, since it is well known that the reflecting power increases but 

 slightly up to this angle. Of course, the assumption is tacitly made 



