GENERAL DISCUSSION OF THE SPECTRA. II5 



SUMMARY. 



The infra-red absorption spectra of organic compounds have been 

 studied, the majority to 15 /u, using a radiometer, two mirror spectro- 

 meters, and a rock-salt prism. Out of a total number of at least 135 

 compounds examined with the rock-salt prism, 131 have been recorded 

 in this paper. They include solids, liquids, and gases. In addition to 

 this, 19 compounds were examined to 2.^ /x, using a quartz prism. 



The following are some of the results obtained : 



( 1 ) A study of isomeric compounds shows that the arrangement, or 

 bonding, of the atoms in the molecule, i. e., its structure, has a great 

 influence upon the resulting absorption spectrum, which agrees with 

 Julius (loc. cit.). 



This is of considerable significance, and is in marked contrast with 

 stereomeric compounds, like dextro- and laevo-pinene, which were 

 found to have identical spectra, showing that the spacial arrangement 

 of the atoms, i. e., the configuration of the molecule, had no effect upon 

 the resulting absorption spectrum. 



(2) No shifting of the maxima of absorption, with increase in molec- 

 ular weight, " Kundt's Law," could be detected, except in the case of 

 the band lying between 3.1 and 3.5 /u,, for gases. Instead of a shifting 

 of the maximum in certain compounds, there occurs a new band beside 

 the original one when a methyl or amido group is substituted for a 

 hydrogen atom, the new band lying toward the longer wave-lengths 

 when a methyl group is substituted, and toward the shorter wave- 

 lengths when the hydrogen atom is replaced by an amido group. This 

 disagrees with investigations of Kriiss in the optical region, where only 

 the new hand was observed. 



(3) A rise in temperature of 20° had no eflfect upon the transparency 

 of the compound, nor upon the position of its maxima. 



(4) The effect of replacing an H atom by certain groups of atoms, 

 like NH, and CH3, is very marked, and usually shows new bands, e. g., 

 2.g6 and 3.43 /x, in the resulting absorption spectrum. In the spectra 

 of certain benzene derivatives, however, the bands of the benzene spec- 

 trum are usually present, showing that the vibration of the benzene 

 nucleus has not been destroyed. However, the writer does not con- 

 sider this sufficient evidence to consider the new bands to be due to the 

 groups of chemical atoms substituted. 



(5) Total absorption is not influenced by the size of the molecule, 

 while compounds having sulphur or halogens are more transparent than 

 those having H, O, OH, or N, which they have replaced, just as found 

 by Friedel and by Zsigmondy. 



