38 INFRA-RED REFLECTION SPECTRA. 



for long wave-lengths. 1 The reflection curves of such metals as gold and 

 silver, however, are entirely different from those of the electrical non- 

 conductors (see fig. 24). In the former the extinction coefficient is always 

 high, while in the latter the extinction coefficient fluctuates through a great 

 range. In quartz the ions have a proper period of undamped electrical 

 vibration which almost totally reflects the wave trains in the region of 

 8.5 and 9.03 n, while for the shorter wave-lengths the vibration periods 

 are more damped, the reflection is enormously decreased, and more of the 

 energy is absorbed. In the case of a reflecting surface composed of elec- 

 trically non-conducting material, of the total energy falling on the surface, 

 the reflected energy in and near the visible spectrum will consist of that 

 reflected from the surface and that part which enters the particles and is 

 returned, due to internal reflection. In the region of 8 to 10 \i (for sili- 

 cates) almost all the observed energy will be reflected from the surface, 

 and hence none will be reemitted due to internal reflection. As a whole, 

 then, what the writer found is that the silicates reflect like "transparent 

 media" for all wave-lengths up to 8// and like metals from 8.5 to 10 fi. 

 In other words, it may be said that in quartz (Si0 2 ) the silicon ions retain 

 the proper period of undamped electrical vibration which they would have 

 in the metal, silicon, for wave-lengths 8.5 and 9.03 p., while, for all other 

 wave-lengths, the vibration periods are more damped and the reflecting 

 power is decreased. 



1 Hagen and Rubens: Ann. der Physik, 8, p. 1, 1902. 



