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The principal chromophores are the azo, -N=N-, azoxy, =N2O, nitro, -NO2, nitroso, -NO, and carbonyl, =CO, groups.

The nitroso group is less important.

The first class includes those substances which require no preliminary treatment, and comprises the amides and ammonium compounds, pyridines, quinolines, alkaloids, albumens and related bodies; the second class requires preliminary treatment and comprises, with few exceptions, the nitro-, nitroso-, azo-, diazo- and amidoazo-compounds, hydrazines, derivatives of nitric and nitrous acids, and probably cyanogen compounds.

The mixture is now placed in a hood and the nitroso dimethylaniline added all at once, or as rapidly as possible.