12 A STUDY OF THE ABSORPTION SPECTRA. 



chrome causes the coefficient of absorption within the band to be smaller. 

 A good example of an auxochrome is that of the group CO C = C CO 

 in indigo: 



.CO, .COs 



C H 4 < >C = C< >C 6 H, 



A similar role is played by the same auxochrome in the deeply colored 

 compounds similar to the indigos: 



.co x co x 



C 6 H 4 ( >C = C< >C 6 H 4 and 



x o/ x o/ 

 .co x co x 



C 6 H 4 <; >C = C< >C 6 H< 



Alkyl, aryl, and the halogens act as bathochromes; while the acylenes 

 CH 3 CO and C 6 H 5 CO act as hypsochromes. Kriiss has shown that CH S , 

 OCH 3 , C 2 H 5 and Br shift the absorption bands to the red, N0 2 and NH 3 

 towards the violet. 



In general, the color, the position of the absorption bands and the 

 extinction coefficients vary for different solvents. In many cases a very 

 plausible explanation is to assume the formation of chemical compounds 

 between the dissolved salt and the solvent. 



Benzene and its derivatives show selective absorption in the ultra- 

 violet. In alcohol the benzene absorption consists of seven bands between 

 X 2330 and X 2710. The absorption and fluorescent spectra of a large number 

 of compounds containing the benzene ring have been investigated. A good 

 example is that of anthracene. This shows the following fluorescent bands. 



Solid A 4250 ; 4495 X 4745 X 4980 A 5300 



Solution X 4050 A 4275 A 4540 A 4820 



Vapor X 3900 X 4150 A 4320 



Benzene gas has some 30 bands. In solution the bands are broad. 



The bands of both the vapor and solution are, in general, shifted to 

 the red when chlorine, bromine, the meth}d group, etc., replace the hydro- 

 gen. The shift is greater the greater the molecular weight of the entering 

 atom or group. The bands of benzene that are shifted are those that are 

 common to benzene, toluene, ethylbenzene, and oxyxylene, and are un- 

 affected by temperature and pressure. 



Hartley gives the following wave-lengths for these bands: 



In solution. J 2682 X 2657-2642 X 2614-2600 X 2539 X 2480 X 2426.5 X 2376 

 In vapor. . . . X 2670 X 2630 X 2590 X 2523 X 2466 A 2411 A 2360 



The substitution products of benzene have much less characteristic 

 spectra than benzene itself. 



THEORY OF DYNAMIC ISOMERISM. 



Baly and others have recently supported the view that the absorption of 

 light by organic compounds does not take place under ordinary conditions, 

 but that absorption takes place when there is a change in the way in which 



