CHAPTER II. 



EFFECT OF SPECIAL GROUPS OF ATOMS OF RADIANT ENERGY. 



In the present chapter it is purposed to discuss some of the relations 

 found among the infra-red spectra of the various substances examined; 

 and incidentally to explain some apparently inconsistent statements made 

 in Part I on the effect of certain particular groups of atoms in producing 

 characteristic absorption bands. 



COMPARISON OF ULTRA-VIOLET AND INFRA-RED. 



Hartley 1 found that an open chain of CH-groups, as well as a union of 

 C and N atoms, produced no absorption bands in the ultra-violet. Neither 

 could he identify any absorption band, of any of these substances, with the 

 carbon atom in the benzene ring; the molecules behaved as though there 

 were no special atoms present. This is just the opposite of what was found 

 in the infra-red. But the length of the infra-red spectrum examined is 60 

 times as long as the ultra-violet. It is, therefore, possible to find relations 

 in the infra-red which, on account of the narrowness of the spectrum, may 

 not be found in the ultra-violet. In the infra-red there are transparent 

 regions between characteristic absorption bands, which are often several 

 times the width of the entire ultra-violet spectrum. The petroleum distil- 

 lates (chain compounds) have a wide transparent region between 4 and 5 [i } 

 just as Hartley found in the ultra-violet. In general, if we were to examine 

 a narrow region of the spectrum, e.g., the ultra-violet, or at 4 /1 in the infra- 

 red, it appears to be a matter of chance whether or not absorption bands 

 will be found there. In the visible spectrum many substances have ab- 

 sorption bands, i.e., are colored. If the eye were sensitive to rays of 

 wave-lengths 3 to 3.5//, all the carbohydrates would appear "colored," 

 and the position of the maximum of the absorption band would be just 

 as irregular as those found in the visible and in the ultra-violet spectrum. 



THE HYDROXYL GROUP. 



During the preparation of the third volume of Kayser's "Handbuch 

 der Spectroscopic" only the writer's "Preliminary Report on Infra-Red 

 Absorption Spectra" 2 was available to the compilers. They call attention 

 to the fact that "the writer found an effect due to special groups of atoms, 

 particularly OH and CH 3 . Although many substances, which contain the 



1 Kayser: Spectroscopy, 3, p. 169 and p. 170. 



2 Coblentz: Astrophysical Journal, 20, 207, 1904. 



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