112 INFRA-RED ABSORPTION SPECTRA. 



in the present work, in which the band was found to oscillate between 

 the values of 1.68 /u, for benzene to 1.74 /t for caproic acid, for ethyl suc- 

 cinate, and for methyl acetate. From this point to 3 /x there are numer- 

 ous small bands of minor importance. Somewhere between 3.1 to 

 3.43 fi a band is found for every carbohydrate studied, oscillating from 

 3.25 fi in benzene to 3,43 fi in the alcohols and compounds rich in CH3- 

 groups. In the region extending from 4 to 5 /x there is great trans- 

 parency and often no strong bands (except in the case of the mustard 

 oils), and for the more complex compounds, e. g., the petroleum distil- 

 lates, there are generally no lines that could be detected even with the 

 large dispersion used. Beyond 5.5 fi the transmission curve decreases, 

 often very abruptly, terminating in strong absorption bands varying 

 from 6.75 yn in benzene, 6.86 ju, in aliphatic compounds, and 7 /* in ter- 

 penes. Beyond this region the bands become stronger, better defined, 

 while the transmission curve varies from great transparency to com- 

 plete opacity. The region at 12 /<, is often lacking in absorption bands, 

 and finally we come to a region of frequently great absorption, with 

 bands occurring from 13.6 fi to 14.2 /a. Beyond this it is difficult to pen- 

 etrate, but all observations made indicate conditions similar to those 

 existing in the region investigated. 



In addition to the general characteristics of the spectra of carbo- 

 hydrates, it will be noticed that the characteristic bands of benzene 

 derivatives are at 3.25 /x, 6.25 ix, and 6.75 fi ; that of the aliphatic com- 

 pounds, e. g., the petroleum distillates, at 3.43 /x, 6.86 /x, and 13.6 /x to 

 13.8 /x; that of carbon tetrachloride at 13 /x; that of compounds having 

 N or NH2 at 2.95 fi and at 6.1 /x to 6.2 /x ; that of the fatty acids at 3.45 /x 

 and 5.86 /ix; that of the alcohols at 2.95 /x and 3.43 /«,; and that of the 

 mustard oils at 4.78 /x. The region of great transparency from 4 to 5 /x 

 is also to be noticed, since the larger dispersion failed to show the pres- 

 ence of strong lines. 



OCCURRENCE OF HARMONICS. 



In discussing the question of the presence of simple relations among 

 the spectral lines in the optical region Cornu^ shows that it is useless 

 to search for harmonic overtones, since the case is relatively rare. He 

 adds that the law of vibration in whole numbers is applicable only to 

 a particular form of sounding bodies, of which the type is a cylindrical 

 column, whose length is great in comparison to the cross-section. In 

 any other type except this special one the relations between the vibra- 

 tion numbers of the successive tones is very complex. In Kayser's 



'Cornu : Compt. Rend., 100, p. 1181, 1885. 



