iazonium chloride with meta-phenylene
diamine. It crystallizes in red octahedra and dyes silk and wool yellow.
Triamino-azo-benzene (meta-aminobenzene-azo-meta-phenylene diamine or
Bismarck brown, phenylene brown, vesuvine, Manchester brown),
NH_2.C_6H_4.N_2.C_6H_3(NH_2)_2, is prepared by the action of nitrous acid
on meta-phenylene diamine. It forms brown crystals which are readily
soluble in hot water, and it dyes mordanted cotton a dark brown. On the
composition of the commercial Bismarck brown see E. Tauber and F. Walder
(_Ber_., 1897, 30, pp. 2111, 2899; 1900, 33, p. 2116). Alkylated
amino-azo-benzenes are also known, and are formed by the coupling of
diazonium salts with alkylated amines, provided they contain a free para
position with respect to the amino group. In these cases it has been shown
by H. Goldschmidt and A. Merz (_Ber_., 1897, 30, p. 670) that the velocity
of formation of the amino-azo compound depends only on the nature of the
reagents and not on the concentration, and that in coupling the
hydrochloride of a tertiary amine with diazobenzene sulphonic acid the
reaction takes place between the acid and the base set free by the
hydrolytic dissociation of its salt, for the formation of the amino-azo
compound, when carried out in the presence of different acids, takes place
most rapidly with the weakest acid (H. Goldschmidt and F. Buss, _Ber_.,
1897, 30, p. 2075).

_Methyl orange_ (helianthin, gold orange, Mandarin orange),
(CH_3)_2N.C_6H_4.N_2.C_6H_4SO_3Na, is the sodium salt of
para-dimethylaminobenzene-azo-benzene sulphonic acid. It is an orange
crystalline powder which is soluble in water, forming a yellow solution.
The free acid is intensely red in colour. Methyl orange is used largely as
an indicator. The constitution of methyl orange follows from the fact that
on reduction by stannous chloride in hydrochloric acid solution it yields
sulphanilic acid and para-aminodimethyl aniline.

_Oxyazo Compounds_.--The oxyazo compounds are prepared by adding a solution
of a diazonium salt to a cold slightly alkaline solution of a phenol. The
diazo group takes up the para position [v.03 p.0082] with regard to the
hydroxyl group, and if this be prevented it then goes into the ortho
position. It never goes directly into the meta position.

The constitution of the oxyazo compounds has attracted much attention, some
chemists holding that they are true azophenols of the type R.N_2.R_1.OH,
while others look upon the