INVESTIGATION WITH A ROCK-SALT PRISM. 79 



The maxima occur in the following order of increasing wave-length : 



[Para, meta, ortho.] 



At 6.8 ,« 

 At gn 

 At 9.7 /i 

 At 13// 



P 



(m),p 



P 



P 



(P) 



Apparently the ortho, in which the CHg-groups are the closest to- 

 gether in the benzene ring, has the greatest shifting of the maxima. If 

 we take the center of gravity of the groups of maxima found by Pauer^ 

 in the ultra-violet, the shift of the long waves is just the reverse, viz, 

 o, m, p. However, we can not place great reliance upon the location 

 of the center of gravity determined in this manner. In cumene, in 

 which the CHg-group is joined to the benzene ring by means of a CH- 

 group, the band lies at 13.4 fi, while in orthoxylene it is at 13.6 /^. The 

 probable significance of this must be reserved for later discussion. 

 With the large spectrometer a trace of the 3.25 /a band is noticeable in 

 the orthoxylene curve, while the 6.2 fi to 6.3 /x band is double, as is also 

 the 6.81 fi band, which is the mean of 6.75 fx and 6.86 /x. This would 

 indicate that the vibration of the benzene nucleus has not been destroyed. 

 The para is still more complex at 6 /x, having an additional maximum 

 at 6.55 fi. These three compounds furnish the best evidence in favor 

 of a resonance efifect of small, electrically charged particles. In the 

 orfJio, which has the two CHg-groups nearest together, the large absorp- 

 tion bands lie farthest toward the long wave-lengths, while in the para, 

 where the CHg-groups are separated the most, the large bands lie far- 

 thest toward the short wave-lengths ; the meta has its bands in inter- 

 mediate positions. This is right in line with the idea that absorption is 

 due to a resonance of small, charged particles, whose capacity, and hence 

 whose period, depends upon their proximity to similarly charged parti- 

 cles ; the closer the particles, the greater the capacity ; hence the slower 

 the period, and hence the farther is the absorption band shifted toward 

 the longer wave-lengths. (See Appendix IV.) 



Mesitylene. C6H3(CH3)2. (Fig. 80.) 

 By substituting three CHg-groups, the 3.25 fi band is 

 entirely obliterated by the one at 3.4 fi. The region at 

 6.18 to 6.3 fi is similar to that of the other methyl deriv- 

 atives. The curve as a whole is very similar to toluene, 

 except at 5,5 fi and the new band at 1 1.95 /i. From the 

 asymmetry of the curve there are indications of the 3.25 fi band of ben- 

 zene. All these methyl derivatives are very transparent. The benzene 



HC 



HCv 



'^ ^C-CH3 

 H 



^Pauer, loc. cit. 



