94 INFRA-RED ABSORPTION SPECTRA. 



formed into a thin, uniform film. Being polymeric, it has a high 

 molecular structure, the constitution of which is not known. The 

 enormous drop in the transmission of 94 to 10 per cent in going from 

 2.5 fj. to 3.5 fx has been found in but few other substances, e. g., the mus- 

 tard oils, at 4.8 /x. The 3.5 fx band is strong and appears complex. The 

 5.94 )u, band and the 14 fx band are well defined. The transparency, in 

 general, is strikingly similar to pinene, except that it becomes more 

 transparent beyond 12 [x. The region from 7 /x to 12 /a is marked by a 

 very sharp band of general absorption. This will be noticed in the 

 following compound. If the selective absorption were dependent upon 

 the number of atoms in the molecule, /. e., its size rather than upon 

 groups of atoms, one would expect to notice their effect in this many- 

 atomed compound. In this connection it is well to notice stearic acid 

 (CigHgeOa, fig. 60), which has but few bands. Unless it can be shown 

 that the absorption bands are complex, then these curves indicate that 

 the cause of absorption is simpler than one would suppose. The sharp- 

 ness of these bands is worthy of notice in connection with other com- 

 pounds containing oxygen. 



The effect of the size of the molecule is not so easily illustrated, since 

 as the number of atoms increases in a molecule there is a tendency for 

 the substance to be solid. Since most of the solids are crystalline, there 

 is great difficulty in obtaining uniform films, free from rills. FriedeP 

 has studied the total absorption of a great many organic compounds, 

 and has found that the absorption does not depend much upon the size 

 of the molecule, but seems to depend upon some property of the com- 

 pound. 



EUCALYPTOL. CioHisO. (Fig. IIS.) 



Eucalyptol belongs to the simpler, monocyclic 



HgC c-^ CH2 terpenes. The first thing one notices in glancing 



9 I . at the curves of this substance is the enormous 

 band of general absorption from 6.5 /x to 14.5 /x. 

 ^ ^ This seems to blot out the bands of selective ab- 



sorption, so that they are not very deep in this region. 



The structural formula shows the O atom joined to two C atoms, 

 while the ring is composed of CHo and CH3 groups. In connection 

 with this, notice the curves of piperidine, which has a ring of CHg 

 groups, and stearic acid, which has a chain of them. Here there is no 

 similarity in the curves. The 3.43 fx band is sharp, while the one at 

 5.9/1, to 6.1 fx is complex. The depression at 14 fx is peculiar, consider- 

 ing the fact that beyond 11 fx this substance becomes more transparent 



^Friedel : Ann. der Physik (3), 55, 453, 1S95. 



HaC C CH2 



