90 INFRA-RED ABSORPTION SPECTRA. 



the opacity in this region is variable, and, since similar cases are found 

 where there is no OH present, it appears that water is the only sub- 

 stance that remains opaque beyond 8 fi. 



The 5.8 fi band is absent, or shifted to 6.2 fi, while the 7 /* band is inde- 

 terminate, as is also the one at 14 fi. It would have been interesting 

 to learn whether there are bands at 7.0 /a and 14 fi, but the great opacity 

 of this substance makes its investigation difficult. The region at 3 /a 

 was examined with the large spectrometer, and appears to be complex, 

 with the largest maximum lying between the benzene and supposed 

 water bands. The chief value in examining these two compounds lies 

 mostly in showing the general transmission of the acetate. The most 

 significant point is their great opacity from 6/* to 10 [i. 



ALDEHYDES. 

 Benzaldehyde. CeHsCHO. (Figs. 106 and 108.) 

 H Here one H atom (figs. 106 and 108) In the benzene 



Hc-^ ^C'^H ""8^ ^^^ ^^^" replaced by the group CHO. The result 

 11 L is a remarkable change in the transmission curve, the 



^C like of which, for depth and sharpness of absorption 



bands, is to be found in but few of the other sub- 

 stances studied. It represents a mode of vibration as free as that of the 

 gas-molecule. In this connection the photographic work of Abney and 

 Festing^ is to be noticed. They found that when O, with H, is com- 

 bined in a radical it causes the spectrum to be linear, rather than bonded, 

 the lines being better defined than when O is more loosely banded. In 

 eucalyptol (CioHigO) the atom is banded to two C atoms, while the 

 transmission curve is quite smooth with shallow minima. This, how- 

 ever, is due in part to the thick film of eucalyptol used. In benzaldehyde 

 the general transmission is about 70 per cent, with a sudden decrease to 

 almost zero at many points, e. g., 12. i fi. The exploration, in which no 

 settings were made, extends to 16 fi, beyond which it was impossible to 

 go, on account of the absorption of rock salt. The 5.84 [x band, so often 

 to be noticed, is very prominent. 



The fact that the O atom is bonded with the C atom, instead of being 

 an OH-group, is to be noticed in connection with the question of the 

 transparency of a compound containing this radical ; also whether the 

 OH-group has a definite efifect in causing certain absorption bands. 



With the large spectrometer the regions examined at 3 yu, and 6 /* 

 were found complex. The benzene bands at 3.25 /a and 6.25 /x come 

 out strong, while the 6.75 /x band is obliterated by the new one at 6.9 fx, 



^Abney & Festing : Phil. Trans., 172, p. 887 (1882). 



