60 INFRA-RED TRANSMISSION SPECTRA. 



OH-group, have an absorption band between 2.9 and 3 p, Coblentz ques- 

 tions whether they are due to the OH-group. On the other hand, he 

 thinks the CH 3 -group causes the band at 3.43 p." 



This inconsistency is easily explained. One need but examine the 

 third volume of Kayser's Spectroscopic, pages 81, 83, 84, 88, 283, etc., 

 and see the numerous exceptions to "Kundt's Law," to be convinced that 

 the announcement of that "Law" was premature. Knowing how subse- 

 quent investigators were misled by this announcement, it seemed to the 

 writer that, so long as there were any serious exceptions, the effect of 

 special groups of atoms on heat-waves should be doubted until the evidence 

 was much greater than obtained at that time. The mineral brucite 

 [Mg(OH 2 )] caused this doubt; for if there is an effect due to the OH group, 

 one would expect to find it in such a simple compound as this one. The 

 first examination of brucite showed no band at 3 p, but a wide band with 

 a maximum at 2.6 p. Since then this mineral has been re-examined, using 

 a larger dispersion, and the large band was resolved into its components, 

 with maxima at 2.5, 2.7, and 3 p. In other words, this mineral has the 

 characteristic band of the hydroxyl group. In the meantime, numerous 

 other substances (see Carnegie Publication No. 65) containing OH-groups 

 and water of crystallization have been examined. In Carnegie Publication 

 No. 65, the exceptions to the rule that the OH-group has a characteristic 

 absorption band at 3 p were found among minerals of which the chemical 

 constitution is in doubt. The absorption band at 3 p, found in substances 

 containing hydroxyl groups, is, therefore, to be ascribed to that group of 

 atoms. The band at 3 p found in the alcohols belongs, therefore, to the 

 OH-group, and one ought not expect (as the writer stated in Carnegie 

 Publication No. 35, p. 108) to find a second band at 6 p (which is found 

 in water), if the 3 p band is due to the OH-group. The OH group 

 apparently does not produce a harmonic series of bands such as are to 

 be found in compounds containing CH 2 or CH 3 -groups. Water also has 

 a harmonic series of bands, but it has not yet been shown to be due to 

 the OH-group. 



THE EFFECT OF MOLECULAR WEIGHT OF THE MAXIMA. 



In Part I a search was made for a shifting of the maximum of an ab- 

 sorption band with an increase in the number of atoms, or especially of 

 groups of atoms, in the molecule. The evidence was very contradictory. 

 In the case of the methyl derivatives of benzene, the maximum of the 

 band was found at 3.25 p in benzene, at 3.3 p in toluene (C 6 H 5 CH 3 ), at 

 3.38 fx in the xylenes [C 6 H 4 (CH 3 ) 2 ], and at 3.4 p in mesitylene [C 6 H 3 (CH 3 ) 3 ]. 

 In other words, by substituting three CH 3 -groups for an H atom, we have 

 shifted the maximum from 3.25 to 3.4/'. Several gases showed a shift of 

 the band, in the region of 3.2 to 3.5 p, toward the long wave-lengths with 

 an increase in the number of H atoms. On the other hand, several nitrogen 



