HISTORICAL. 9 



tion band is shifted to the red, while by the introduction of a nitro 

 (NO2) or an amide (NH2) group the band is shifted toward the violet. 



Subsequent writers on this subject always mention the shift toward 

 the red zvith increase in molecular weight, but rarely mention the fact 

 that there is also a shift in the opposite direction; so that, unless one is 

 familiar with the original work, the quotation is misleading. In quoting 

 such a complete investigation which records two well-defined series of 

 phenomena, apparently opposed to each other when considering the ques- 

 tion of molecular weight, it seems highly desirable to have the complete 

 observation rather than the part which fits the particular problem under 

 investigation. This is especially desirable in work like that of Ranso- 

 hoff, who thought that a small sharp band found at 4.9 fx. in CH3OH 

 was shifted to 5.2 /x in C2H5OH, " which would be an example like that 

 of Kruss." He found no shifting for larger bands. 



The following is what Kriiss (loc. cit.) observed for indigo: 



I Indigo in CHCI3 "^^ max. at 0.6048 jn 



Shift to red. . •, Methyl indigo in CHCI3 ^- max. at 0.61917 



[ Ethyl indigo in CHCI3 /" max. at 0.6526 



„, .J ., I Indigo in CHCI3 / max. at 0.6048 



Shift to violet -( ^,. . ,. . ^,,^, . 



Nitro-indigo in CHCI3 /- max. at 0.5858 



This is a shift of 0.0055 P- P^^ atom of Br, which proportionality was 

 found not to hold true. 



E. VogeP found that the occurrence of chlorine in the meta-position 

 in the " Carboxylrest " in fluorescein shifts the absorption maximum 

 more toward the red than when it is in the ortho-position. As with 

 Kriiss, he found that the more hydrogen atoms that have been substi- 

 tuted, the greater is the shift of the maximum of the absorption band, 

 but that the shift is not proportional to the number of hydrogen atoms 

 substituted. 



The fluorescence and absorption of dyestuffs in solution and in solid 

 gelatin was investigated by Stenger.^ He concludes that the absorption 

 of light depends primarily upon the size of the physical molecule, and 

 it is only when a change in the aggregation conditions, or in solving 



^E. Vogel ; Ann. der Physik (3), 43, p. 449, iJ 

 ^Stenger : Ann. der Physik (3), 33, p. 578, 18J 



