82 INFRA-RED ABSORPTION SPECTRA. 



The first maximum at 3.43 fi is to be noticed in considering the possi- 

 biUty of its being due to CH3 ; also the 6.86 fx, which appears to be har- 

 monic with it. The 5.8 jit band found prominent in so many compounds 

 is shifted to 6.0 )u, as in the terpenes. As a whole the benzene is 

 changed so that there is only a slight indication of the benzene band at 

 3.25 ti and at 6.25 /*. In this connection it is well to notice toluene and 

 mesitylene, which have fewer CHg-groups. The upper curve was 

 obtained with a narrow slit, 0.4 mm., and a film, o.oi mm. in thickness. 

 It shows that the general transmission is on a line of about 80 per cent. 

 It also brings out the fact that the greater opacity beyond 6 ^ is due to 

 the numerous absorption bands, which, by overlapping, lower the whole 

 curve when a thicker film is used. Of still greater interest is the fact 

 that the 6.86 fi and 7.3 [j. bands occur in the petroleum distillates, which 

 are chain compounds. The latter predominate in CH^-groups and gen- 

 erally have two CHg-groups, but whether we are to attribute them to 

 the former is difficult to decide. The similarity in the curves of cymene, 

 pinene, and limonene is a strong argument in showing that the cyclic 

 structure is not the only cause of the characteristic spectra of the 

 terpenes. 



Cyanine. C29H35N2I. (Fig. 89.) 



The chemical constitution of cyanines is unknown. Nietzki^ says 

 of it that "possibly the cyanines possess a structure analogous to that 

 of the phenylmethane dyestufifs," e. g., fuchsine, C20H19N3HCI+4H2O. 

 The absorption spectra of these two compounds had previously been 

 investigated by me^ to 10 /a and found unusually similar. Hartley and 

 Dobbie' found that alkaloids having similar structure give similar ab- 

 sorption spectra, showing that it is only in the details that the structure 

 is different. The same is true in the present work on the petroleum 

 distillates. The band at 3.43 fi occurs in the usual place for compounds 

 having CHg-groups. Whether the benzene nuclei have any effect is 

 difficult to decide, although the 6.7 ix and 8.7 /a bands are close to those of 

 benzene. The broad regions of opacity at 7 ju, and 13 /* are very marked. 



BenzonitrilE. CsHsCN. (Fig. 90.) 



The result of the substitution of a €N-group for an H atom in ben- 

 zene has produced an absorption spectrum of an unusual character. Its 

 general transmission is about 80 per cent throughout the spectrum. 



There are no deep absorption bands, except one of 80 per cent at 



^Nietzki : Chemie der Organischen FarbstoEfe, p. 262, 1901. 

 *Phys. Rev., vol. xvi, p. 119, 1902. 

 ^Hartley & Dobbie, loc. cit. 



