INVESTIGATION WITH A ROCK-SALT PRISM. yj 



which makes it appear that the facts gained by chemical and physical 

 analysis are intimately related. 



Benzene shows a great transparency throughout the whole spectrum 

 to 14 /x, where it suddenly becomes opaque. The well-defined trans- 

 mission minima are seven in number. The 7.3 /a band found by Julius^ 

 occurs only as a slight depression, and the possibility of its being due 

 to thiophene, which has a strong band in this region, is mentioned in the 

 discussion of that substance. This sample was known to be free from 

 thiophene. Certain regions of the spectrum were re-examined thor- 

 oughly, but no new transmission minima could be found. The shorter 

 curves show slight depressions sometimes, but no deep bands occur in 

 them. The cell containing the benzene was dismounted and stood aside 

 for two weeks, but on examination no change was detected in the 

 maxima. The curve found was so similar to the first that it was quite 

 evident that the benzene had not attacked the glue of the cell in the 

 meantime. The complex band at 9.8 /x to 10.3 \i is to be noted, also the 

 1 1.8 /i, band, which is in common with other compounds ; also the 3.25 \i. 

 and 6.75 /A bands, which occur even in very complex derivatives, show- 

 ing that the vibration of the benzene ion is not destroyed. 



In benzene and its derivatives the 5.8 ju, band is not to be found except 

 in those containing CHg-groups and in benzaldehyde. Whether this 

 band is due to CH3 or some group of atoms vibrating like a CH3- 

 group is a pertinent question. 



The slight band at 6.25 /i is to be noticed, since in certain derivatives 

 it is very strong, just as though the vibrations were less damped. This 

 is the only example found in the whole research. 



METHYL DERIVATIVES OE BENZENE. 



Toluene. CeH^— CH3. (Fig. 79.) 



p.p„ By studying a group of benzene derivatives, increasing 



HC^ ^CH in the number of CHg-groups, it was hoped that the effect 



HcL CH °^ those groups could be determined, as well as the shift- 



^^^ ing of an absorption band, with increase in molecular 



weight. 

 The introduction of a CHg-group has shifted the 3.25 fx band of ben- 

 zene to 3.34 fi. The one at 6.25 /x has been strengthened, while the 

 6.75 /x is quite obliterated by the 6.86 fi maximum found in the petroleum 

 distillates and other chain compounds. This is shown to better advan- 

 tage in the curves obtained with the large spectrometer, which also 

 show fine bands at 5/^, not detected with the smaller apparatus. Whether 



^Julius, loc. cit. 



