INVESTIGATION WITH A ROCK-SALT PRISM. 69 



When a thinner film was examined the band at 6.5 ix was found 

 double, while with the large spectrometer the region at 3 to 4 /a was 

 found a complex of three bands. 



This is the most important compound investigated, since it shows 

 the superposition of the spectra of benzene and the mustard oils. We 

 have not only the maxima of benzene at 3.25, 6.25, 6.75, 8.7, and 10 /x, 

 but also the marked band at 4.8 /a, which is common to all the mustard 

 oils. This band at 4.8 /* in phenyl mustard is probably the strongest 

 argument we have in favor of the eflfect of a group of atoms, like CS, 

 in causing absorption. Here the NCS is bonded to a C atom, just as 

 in the preceding compounds, and we would expect it to produce the 

 same effect. In addition to this, however, we have the vibration of the 

 benzene nucleus, or ion, and it seems the simplest to think of the result 

 as being due to the resonance of the two kinds of ions. 



THE FATTY ACIDS. 



This group of compounds forms a distinct class by itself. They can 

 be obtained by oxidation of the primary alcohols. In fact, the consti- 

 tution of the alcohols, ethylene, the aldehydes, etc., has been derived 

 from that of the fatty acids. Hence we would expect to find physical 

 relations between them. The first thing to be noticed in the curves is 

 the prominence of the 5.8 /a band, which is weak in the alcohols, and 

 the transparent region at 9.5 /x, where the alcohols have a large absorp- 

 tion band. The regions of 3.5 /a, from 7 to 8 /t, and of 14 jx are similar 

 for the alcohols and the acids. 



The fatty acids were studied in connection with the question of the 

 effect of an OH-group in the molecule. In the primary alcohols, 

 R — CH2OH, the OH is joined to a carbon atom, while in the fatty acids, 

 R — iCO — OH, the OH is joined to an oxygen atom. Another distinc- 

 tion is that in electrolysis, or the formation of an ester, the OH of the 

 alcohol is separated as a hydroxyl, while in the acids it is simply the 

 H atom that is replaced. 



The question of the effect of the OH-group will be discussed sepa- 

 rately, and it will be sufficient to add here that the 2.95 {x band, which is 

 characteristic of the alcohols and is suspected to be due to OH, is not 

 found in the fatty acids. 



Acetic Acid (Glacial). CzHiO-. (Fig. 55.) 



This compound is of interest on account of the difficulty to free it 

 from water. From its curve and that of pure water one would infer 

 that the acetic acid was free from water. It has three regions of great 

 opacity, viz, 3.5 ^, 6 to 8 fx, and 10 to 12 fx, while at 15 yu, it is fairly trans- 



