190 Dr. Carnelley on the Influence of Atomic Arrangement 



The strength of the fluorescence is very different for different 

 members of the fluorescing series. Thus, /S-anthracene disul- 

 phonic acid is strongly fluorescent, whilst the a-acid is but 

 slightly so. 



VII. Influence of Atomic Arrangement on the 

 Tinctorial Properties. 



The influence of chemical constitution on the tinctorial pro- 

 perties of compounds has been studied more especially by Otto 

 Witt (Ber. ix. p. 522), whose more important conclusions 

 may be briefly stated as follows : — 



Since benzene is not a dye-stuff, it follows that colouring- 

 properties are conditioned by the side chains. Of the three 

 side chains, N0 2 , NH 2 , OH, not one of them alone is capable 

 of producing a compound possessing tinctorial properties ; for 

 C 6 H 5 .N0 2 , C 6 H 5 .NH 2 , C 6 H 5 .OH are all colourless bodies. 

 It is possible, however, that certain combinations of two or 

 more of these groups will produce the necessary result ; thus, 

 of the six possible compounds, 



NO, 



Nil 



only 2 and 3 are dye-stuffs. We therefore see that the combi- 

 nation of the nitro-group with the amido-, or hydroxy! groups, 

 produces a colouring compound ; or, in other words, the pre- 

 sence of a N0 2 and a salt-forming group conditions the dyeing 

 properties. The nitro-derivatives of benzene are to a certain 

 extent coloured ; but it is only on the introduction of a salt- 

 forming radical that we get a true dye-stuff. That to obtain a 

 dye-stuff the presence of a N0 2 group and a salt-forming 

 radical is necessary, is shown by the fact that if we destroy 

 the basicity of an amido-group by the introduction of an acetyl 

 group, or if we destroy the acidity of a hydroxyl group by 

 introducing a methyl group, the compound entirely loses its 

 colouring-properties, though by these changes no alteration 

 has been produced in the general constitution of the com- 

 pound. Thus 



