tached to the nitrogen atom, one of these being in the quinoid form 

 and hence adding a second chromophore. The typical indamin 

 formula is: 



HN_/~\_N /"~\ NH 



In the thiazins, such as methylene blue, the two benzene rings are 

 further joined together by a sulfur atom, forming three closed 

 rings of atoms. The simplest thiazin base would be: 



_S_ /\_NH 

 ACID CHROMOPHORES 



1. The nitro group, — NO2, as in picric acid. 



2. The quinoid benzene ring. 



which occurs in a long series of dyes, such as the indamins above 

 mentioned, the xanthenes and the di- and tri-phenyl methanes, 

 which include many well known stains, such as rosolic acid, 

 fuchsin, methyl green and the methyl violets. A typical triphenyl 

 methane formula is that of pararosanilin, base : 



C^/~"\^NH 



LEUCO COMPOUNDS 



The different chromophores differ considerably from one another, 

 but they all have one property in common. In the language of 

 chemistry, they all have unsatisfied affinities for hydrogen; or in 

 other words, they are all easily reduceable, for combining with 

 hydrogen is the opposite of oxidation and is, therefore, reduction. 

 The nitro group may be reduced to an amino group; in the azin 

 group the bond between the nitrogen atoms may break and two 

 hydrogen atoms be taken on; while in the various chromophores 

 with double bonds (such as the quinoid ring) the double bond may 

 break and hydrogen atoms become attached to the valences thus 

 freed. 



Now in every case this reduction destroys the chromophore 

 group, and as a result the compound loses its color. In other words 

 a dye retains its color only as long as its affinities for hydrogen are 



18 



