INVESTIGATION WITH A ROCK-SALT PRISM. 89 



the bands here, due to the presence of the O atom, has been noticed in 

 benzaldehyde. The region of 3 /a is complex, and when examined with 

 the large spectrometer shows bands at 2.9 fx and 3.4 /*. Whether the 

 3.25 fi band of benzene is still present could not be determined. The 

 indications at 6.26 fi and 6.75 fi are that the benzene ion still manifests 

 itself. The great opacity from y /x to 8 fi is found in thymol and numer- 

 ous other compounds having oxygen or a hydroxyl group. 



ACETYL-EUGENOL. C12H14O3. (Fig. IO3.) 



This compound differs from the preceding in the substitution of 

 COOCH3 for the OH radical. The effect on the eugenol curve is not 

 very marked either in the general absorption or in the transmission 

 minima. The greatest change is from 5 /«, to 6 /x, where the absorption 

 is increased, producing a new band at 5.8/x,. The region from 11 to 

 12 n is also changed, a band appearing at 12.3 fi, while the one at 12.5 fi 

 is barely visible. The whole seems to indicate that the OH-group can 

 not have seriously affected the transmission of infra-red radiation. In 

 fact, the great opacity beyond 8 fx, thought by some investigators to be 

 due to the OH radical, is in the present work, to be found only in water. 

 Three atoms of oxygen are present in this compound, but the maxima 

 are not so sharp as in benzaldehyde, in which the O atom is quite free, 

 being bonded with only one C atom. An unusual arrangement of the 

 bands in pairs is to be noticed in this compound. 



AROMATIC ACIDS. 

 Phenyl Acetate. CeHs— OH:r-COOH. (Fig. 104.) 



The aromatic acids are in many points analogous chemically to the 

 fatty acids. But generally no such similarity is to be observed in their 

 transmission curves. An exception is methyl acetate. There are great 

 similarities in the present and the following compound. For example, 

 they have an opaque region from 6 /a to 9 /n, followed by a more trans- 

 parent region. However, there are no important absorption bands in 

 common. The 5.8 /x band is present in this compound, but is absent in 

 the following one. The 7.0 /a to 7.4 /x band appears complex. The 

 great opacity from S fi to g fi seems to be due to a large transmission 

 minimum, the maximum of which lies at about 8.5 [i. 



Methyl SAUCYI.ATE. CHi^OjC— CaHi— OH(o). (Fig. 105.) 



This ester of the aromatic acids, familiar as oil of wintergreen, has 

 a region of great absorption extending from 6 fi to 9 /a, which is fol- 

 lowed by a transparent region that ends in opacity at 13 /t. The prob- 

 ability of this opacity being due to the OH radical will be noticed else- 

 where. Other examples, like acetic acid and ethyl alcohol, show that 



