COLOR AND CHEMICAL CONSTITUTION 129 



going various pulsations, such as may be illustrated by the accom- 

 panying figures : 



/ V \ 



(a) 



HC " iCH HC-C-CH H C /| >ci 



(b) ! i (c) 



vY 



X c^ H T 



H a 



The clotted lines show the linkings developed from the free affinities 

 when the ring is pulsating between the two forms (a) and (6). The 

 centric formula for benzol (Baeyer's), as shown in (c), may be, there- 

 fore, an intermediate form for all the possible forms. The free or 

 residual affinity possessed by each carbon atom asserts itself under the 

 various conditions which can be brought into existence by these pulsa- 

 tions, with the effect that the several linkings produced must involve 

 always a pair of carbon atoms and then in turn during the second stage 

 of the pulsation must suffer a break and consequently give rise to 

 some particular one of the seven possible phases, with its characteristic 

 absorption band of course depending upon the carbon atoms in ques- 

 tion. Altogether, when the entire ring is free to pulsate in every 

 direction, there will arise seven absorption bands which represent the 

 seven possible combinations of linking-change. 



The derivatives of benzol may be expected to show some variation 

 in type and manner of pulsation from that of the parent ring, but 

 whatever changes occur the effect upon the characteristic absorption 

 spectrum of the original molecule will always indicate the exact nature 

 of each change. In this connection it will be well to consider a few of 

 the more important derivatives, which, as is generally known, are 

 primarily formed by the replacement of one or more of the hydrogen 

 atoms by an equivalent atom or group of atoms — a process called sub- 

 stitution. The alkyl radicals (methyl, ethyl, etc.) stand as a type of 

 the neutral groups and consequently, when they are present, little 

 change in the spectrum of the original substance should be observed. 

 The spectrum of toluol, C 6 H 5 ■ CH 3 , ethyl benzol, C 6 H 5 ■ C 2 H 5 , etc., are 

 almost identical, but only the first two absorption bands of the original 

 benzol spectrum are well marked, the remaining bands having fused 

 more or less into one broad band. With aniline, CgHglSrHj, where 

 the basic unsaturated amido-group (NH 2 ) has replaced the hydrogen 

 atom, we get only a broad absorption band caused, no doubt, by the 

 residual affinity of the nitrogen atom which binds or holds all the 

 free affinities of the benzol ring. Upon the addition of an excess of 

 hydrochloric acid to aniline, we obtain the saturated compound known 

 as aniline hydrochlorate, C 6 H 5 • NH 3 C1, the nitrogen having passed 

 from the trivalent to the quinquivalent state. This compound, as 



vol. lxxii. — 9. 



