6 LankshEAR, Quantitative Absorption Spectra. 



some qualification. In the first place it is true only of 

 complicated bodies, and then only when the substituent 

 is optically inert, e.g. when —OH becomes — OMe. 



3. Persistence and reactivity are closely related. This 

 is well brought out in the substituted benzoquinones, 

 where decrease in persistence is accompanied by a corre- 

 sponding decrease in the ease of oxime formation. In 

 the tetra substituted quinones the quinone band dis- 

 appears and no oximes can be form-ed. 



4. Definite groups tend to give their band always at 

 the same dilution and wave length. Thus the a-diketone 

 band is just at the edge of the visible spectrum and is 

 given in N/io solutions. These substances are thus 

 characterised by their yellow colour. The difficulties 

 surrounding the subject of colour will be appreciated 

 when it is remembered that very many yellow bodies 

 exist which cannot have an a-diketonic structure. 



5. Finally, both selective absorption in the ultra- 

 violet and ordinary visible colour are connected with the 

 existence in the molecule of one or more centres where 

 the chemical potential, or reactivity, is high. 



The above statements, although subject to many 

 reservations, show the nature of the considerations which 

 affect the study of absorption spectra. 



Various theories have been put forward of the origin 

 of selective absorption in organic compounds based on 

 the qualitative curves which the Hartley method gives, 

 but it is highly desirable that there should be accumulated 

 a large body of data derived from a more scientific method. 

 Certain criticisms to which the former method is open 

 may be made under the following heads : {li) source of 

 light, ib) exposure, {c) absorption of light by other than 

 the substance being studied, {d) change in constitution on 

 dilution, {e) difficulties in reading the plate. 



