GENERAL DISCUSSION OF THE SPECTRA. Ill 



In the present work on dextro-pinene and laevo-pinene the maxima of 

 the bands coincide, showing that the shape of the molecule has no influ- 

 ence on absorption spectra. 



GROUPING OF THE SPECTRA. 



Abney and Festing (loc. cit.) say their results indicate "without much 

 doubt that the substances we have examined can be grouped according 

 to their absorption spectra, and that such a grouping, as far as we have 

 examined it, agrees, on the whole, with that adopted by chemists." In 

 other words, certain great groups of compounds have characteristic 

 absorption spectra. This is just what Hartley and Dobbie (loc. cit.) 

 observed for the alkaloids, in which the ultra-violet absorption spectra 

 vary only in minor details. Aschkinass^ found that the minerals con- 

 taining Ca, e. g., fluorite, calcspar, marble, and gypsum, have a band of 

 metallic reflection (absorption) in the region of 30 /a. 



These observations apply to the present work. For example, the 

 terpene group of compounds has a series of bands which is common to 

 all of the compounds belonging to this group. In the same manner the 

 general trend of their absorption curves is similar. The petroleum 

 distillates have all the principal bands in common. The spectra of the 

 fatty acids are conspicuous for the lack, but great depth, of their 

 absorption bands. A glance at their line spectra in Table III gives 

 ample proof of the foregoing statements.^ Having observed that certain 

 groups of spectra are similar, one would naturally search for certain 

 characteristic absorption bands in them ; and, reasoning from this stand- 

 point, we may possibly be able to locate the group of atoms which 

 causes the band. Thus the 2.95 fx band and the OH-group of atoms 

 are characteristic of alcohols, while the 4.78 fi band and the NCS radical 

 are characteristic of the mustard oils. But this avails us little, for a 

 great many other facts, besides the group of atoms, serve as character- 

 istics of these groups of compounds. 



CHARACTERISTICS OF THE SPECTRA OF CARBOHYDRATES. 



The absorption spectra of carbohydrates are conspicuous for the 

 recurrence of absorption bands in certain regions of the spectrum. 

 The first of these regions was found by Abney and Festing (loc. cit.) 

 at 0.74 fj. and 0.867 fi. Puccianti (loc. cit.) found that in all cases where 

 the carbon atom is joined directh' to the h}drogen atom in the molecule 

 the absorption spectrum shows a band at 1.71 ^u. This has been verified 



^Aschkinass : Ann. der Physik (4), i, p. 42, 1900. 

 ^The tables will be found at the end of this volume. 



