INVESTIGATION WITH A ROCK-SALT PRISM. 7 1 



IsocAPROic Acid. (CH3)2CH(CH2)3C00H. (Fig. 58.) 

 The curve of this isomer is identical with that of caproic acid up to 

 Sfi. The first new bands are found at 9.08/1 and 12.15 /a, while the 

 13.8 fi band of caproic is absent. The isomer seems a little more trans- 

 parent, and the large band of caproic at 8 /t is resolved into two bands. 



Oleic Acid. CisHmOz. (Fig. 59.) 

 The sample used was colorless, showing that it had not decomposed. 

 This is an unsaturated acid, CnHoD.sOa, but whether the increased 

 transparency is due to this cause or to the increase in the number of 

 CH2 groups is an unanswered question. Except for a few minor details, 

 the curve is similar to those of the saturated acids, CnHonOg. An exam- 

 ination with the large spectrometer failed to locate new bands except 

 in the region at 3.45 fi, where there are three. The band at 7 /i shows 

 complete opacity from 6.9 fi to y fx. No band could be detected at 3 ft. 

 A drop of water was added to several grams of liquid. The emulsion 

 was examined, but no water band could be detected at 3 /t. This is of 

 interest in the question of the effect of an OH-group. 



Stearic Acid. CiTHseOz. (Fig. 60.) 

 The solid films for this compound were 0.07 and 0.02 mm. in thick- 

 ness. The curves are marked for their great transparency interspersed 

 with four deep, narrow absorption bands. This high transmission in 

 certain regions is to be noticed in other compounds. It is no doubt a 

 characteristic of this acid, although the transparency might have been 

 enhanced by its crystalline condition, since the film was very thin and 

 not continuous. The absence of absorption bands is very significant, 

 and shows that molecular weight and the number of groups of atoms 

 are not of great importance in causing selective absorption. 



Cerotic Acid. CssHsoOj. (Fig. 61.) 

 This solid is obtained from beeswax, and, although we do not know 

 its exact molecular weight, it is high enough to show that absorption 

 does not depend upon that question, as will be seen in comparing it with 

 valeric acid, in which the film was the same thickness. The discussion 

 of stearic acid applies to this compound, since the spectra are the same, 

 except that in cerotic acid there are new bands at 11, 11.65, ^^^ 12.85 M- 

 All the fatty acids are chains of CH, and CH3 groups, like the petro- 

 leum distillates. They differ in composition only in having two addi- 

 tional O atoms. The result is that we have only the 3.45 and 13.8 /i 

 regions in common. The absorption bands are broad and deep in the 

 acids, while they are shallow and narrow in the oils. The 7 ix band of 



