ame hand; the _Roteiro_ of Vasco da Gama's First Voyage (_Journal of
the First Voyage of ... Da Gama_, Hakluyt Soc., ed. E. G. Ravenstein
(1898), pp. 9, 14); Ramusio, _Navigationi_ (3rd ed.), vol. i. fol.
144; Castanheda, _Historia_, bk. i. ch. 1; Galvano, _Descobrimentos
(Discoveries of the World)_, Hakluyt Soc. (1862), p. 77; E. G.
Ravenstein, "Voyages of ... Cao and ... Dias," in _Geog. Journ._
(London, December 1900), vol. xvi. pp. 638-655), an excellent critical
summary in the light of the most recent investigations of all the
material. The fragments of Diaz's only remaining pillar (from Diaz
Point) are now partly at the Cape Museum, partly at Lisbon: the latter
are photographed in Ravenstein's paper in _Geog. Journ._ (December
1900, p. 642). (C. R. B.)

DIAZO COMPOUNDS, in organic chemistry, compounds of the type R.N.2.X
(where R = a hydrocarbon radical, and X = an acid radical or a hydroxyl
group). These compounds may be divided into two classes, namely, the
true diazo compounds, characterized by the grouping - N = N -, and the
diazonium compounds, characterized by the grouping N:.N<.

The diazonium compounds were first discovered by P. Griess (_Ann._,
1858, 106, pp. 123 et seq.), and may be prepared by the action of
nitrous fumes on a well-cooled solution of a salt of a primary amine,

C6H5NH2.HNO3 + HNO2 = C6H5N2.NO3 + 2H2O,

or, as is more usually the case (since the diazonium salts themselves
are generally used only in aqueous solution) by the addition of a
well-cooled solution of potassium or sodium nitrite to a well-cooled
dilute acid solution of the primary amine. In order to isolate the
anhydrous diazonium salts, the method of E. Knoevenagel (_Ber._, 1890,
23, p. 2094) may be employed. In this process the amine salt is
dissolved in absolute alcohol and diazotized by the addition of amyl
nitrite; a crystalline precipitate of the diazonium salt is formed on
standing, or on the addition of a small quantity of ether. The diazonium
salts are also formed by the action of zinc-dust and acids on the
nitrates of primary amines (R. Mohlau, _Ber._, 1883, 16, p. 3080), and
by the action of hydroxylamine on nitrosobenzenes. They are colourless
crystalline solids which turn brown on exposure. They dissolve easily in
water, but only to a slight extent in alcohol and ether. They are very
unstable, exploding violently when heated or rubbed. _Benzene diazonium
nitrate_, C6H5N(NO3):.N, c