Manchester Memoirs, Vol. Iviii. (1914), No. 15. 5 



The curve of selective absorption has one or more 

 maxima and minima, and the portions of the curve about 

 a minimum are said to form an absorption band and the 

 minimum itself is called the head of the band. The 

 vertical distance from a minimum to its adjacent maxi- 

 mum is termed the persistence of the band, and is the 

 range of dilution over which the band occurs. It thus 

 forms a rough measure of the strength of the absorption. 



Generalisations. Certain conclusions have been arrived 

 at which will probably continue to hold good for some 

 time, although the use of more scientific methods may be 

 expected to give them a somewhat different form. 



The following summary, though not complete, will 

 give some idea of their nature. 



1. Aliphatic bodies give general, aromatic give selec- 

 tive absorption. Numerous exceptions exist on both 

 sides. For instance, aliphatic compounds containing the 

 group — CO.CHj — , in which keto-enol tautomerism to the 

 form — COH = CH — is considered to occur, show selec- 

 tive absorption, especially in their metallic derivatives, 

 such as — CONa = CH — , The same is true of aliphatic 

 a-diketones containing the group — CO. CO — . In this 

 case it is supposed that a reversible change to the form 



-C = C- 



I I 

 0-0 



occurs. As regards aromatic compounds, the case of 

 hexahydrobenzene has been already referred to. This 

 may, however, be regarded almost as an aliphatic body, 

 but the general absorption of aniline, CgH6NH2, is not 

 capable of the same explanation. 



2. Substances having analogous constitutions give 

 similar absorption spectra. This statement, first put for- 

 ward by Hartley in connection with the alkaloids, needs 



