14 INFRA-RED ABSORPTION SPECTRA. 



preted, indicates a shift toward the long wave-lengths, as demanded 

 by Kiindt's law. 



Finally, we have to consider the total absorption of an extended series 

 of carbon compounds by FriedeP and by Zsigmondy,^ using Tyndall's 

 method, substituting a bolometer for a thermopile. They found that 

 the transparency of a compound increases if, other conditions being 

 equal, H, O, OH, or N are replaced by S, or halogens. Absorption 

 does not depend upon the size of the molecule. The carbon atoms have 

 little influence on absorption. As a whole the absorption of radiant 

 heat depends upon the manner of the bonding of the atoms in the mole- 

 cule, as well as upon the kind of compound in which the atoms of an 

 element are united. If an amido group occurs in a carbohydrate it 

 outweighs the hydroxyl, e. g., an NH group substituted for an H atom 

 in benzene reduces the intensity to one-fifth its original value. This 

 opacity of nitrogen compounds will be noticed in the present work, 

 where it will be shown that not all nitrogens are highly opaque. 



^Friedel : Ann. der Physik (3), 55, p. 453, 1895. 



^Zsigmondy : Ann der Physik (3), 49, p. 531, 1893 ; 57, p. 639, 1896. 



