588 



REPORT — 1903. 



thirdly, those spectra which exhibit selective absorption, and ■which at the same time 

 exert great absorptive power, or, in other words, can undergo great dilution before 

 the absorption bands are rendered visible, and still further dilution before they 

 are extinguished or obliterated. 



Spectra of the First Variety belong to substances which are constructed 



on 



an open chain of carbon atoms, thus : C • C • C • C • C or CziC • C • C • C and 



c=c-c-o-c. 



The introduction in place of one or more atoms of hydrogen — of hydroxyl, OH, 

 carboxyl, COOH, methoxyl, OCH3, CO, COH, or NH.,, or of side chains such as 

 OH3, CoHj,, &c. — does not affect the character of the spectra, but merely the 

 absorptive power, which is increased when oxygen or an oxygenated radical is 

 introduced. 



Spectra of the Second Variety are spectra of substances so constituted that the 

 carbon atoms form a closed chain. It is immaterial whether the closed chains are 

 homocyclic or heterocyclic ; thus : — 



They possess greater absorptive power than open-chain compounds, but do not 

 exhibit absorption bands. It is manifestly the chain or ring structure of the 

 compounds that gives them greater absorptive power, and not the number of 

 carbon atoms in the molecules. 



Spectra of the Third Variety. — These show absorption bands, and the substances 

 yielding them are generally constituted on the type of benzene, naphthalene, 

 anthracene, phenanthrene, &c. ; but the rings may be either homocyclic or hetero- 

 cyclic without the character of the spectra being altered ; thus : — 



C C CC C CCN 



/ '^ /i\ ^\/^ /=^ -^\/\ /^ 



c c 



\^ 



c 



Benzene. 



I ■" I 



\V 



c 



Benzene. 



c 



c 



c 



c 



If we say that the compounds which are homocyclic are constituted of at least 

 three pairs of carbon atoms doubly linked, which are themselves singly linked 

 together, we may make use of the formula of Kekule for benzene as the simplest 

 expression of their constitution ; if we assume that each of the six atoms is linked 

 to at least other two atoms we adopt what is practically the prism formula of 

 Ladenburg, or the same idea expressed in space of two dimensions. It is difficult 

 to express the physical condition by the Armstrong-Baeyer formula or centric 

 arrangement because this does not clearly suggest to one's mind what is manifestly 

 the fact, namely, that the carbon atoms in the nucleus of benzene are much more 



