These substances are formed from the aromatic amines by treatment with nitrous acid at low temperatures. Primary amines all react with nitrous acid at moderate temperatures when the amino group (NH2) is replaced by a hydroxyl group (OH); thus ethylamine (C2H5NH2) reacts with nitrous acid (HNO2), yielding alcohol (C2H5OH), C2H5NH2 + HNO2 - C2H5OH + N2 + H2O. The aromatic amines, however, if treated below 0° C. with nitrous acid, yield diazo compounds, and not hydroxy compounds, e.g. aniline treated with nitrous acid in hydrochloric acid solution yields diazo-benzene chloride, C6H5NH2·HCl + HONO = C6H5 - N:N·Cl + 2(H2O). Diazo salts are crystalline compounds soluble in water, sparingly soluble in alcohol, and are unstable, decomposing explosively if struck or suddenly heated. In solution in water they decompose as the temperature rises, liberating nitrogen, and forming hydroxy compounds, C6H5N:NCl + H2O = C6H5OH + N2 + HCl. Diazo salts are valuable in the synthesis of different classes of compounds, as the - N:N - group reacts readily with other groups. For this purpose it is usually only necessary to prepare a solution containing the diazo compound. In the preparation of the azo dyes the starting-point is a primary amine; the amino group is 'diazotized', i.e. treated with a solution of sodium nitrite and dilute mineral acid at low temperature. A diazo salt is formed, and is then made to react in solution with a hydroxy compound or an amino compound, &c., with the formation of a highly coloured azo compound. The diazo compound is then said to be coupled. Diazo compounds are therefore important intermediate substances in the manufacture of azo dyes.
The diazo group may also be exchanged for the hydroxyl group by warming the solution with water, or for the cyanogen group by warming with a solution of potassium cyanide, e.g. diazo-benzene chloride warmed with potassium cyanide solution is converted into the nitrile of benzoic acid, C6H5 - N:NCl + KCN = C6H5CN + KCl + N2; or transformed into halogen derivatives of hydrocarbons by warming with cuprous chloride, e.g. diazo-benzene chloride is transformed into chlorobenzene, C6H5 - N:NCl + Cu2Cl2 = C6H5Cl + N2 + Cu2Cl2. These reactions, where nitrogen is eliminated from the compound, and a group or element replaces the two atoms of nitrogen, afford a means of synthesizing a variety of compounds.
Dib´din, Charles, an English dramatic manager and poet, composer and actor, born in 1745, died in 1814. At the age of fifteen he made his appearance on the stage, and was early distinguished as a composer. He invented a new kind of entertainment, consisting of music, songs, and public declamations, which he wrote, sang, composed, and performed himself, and by this means succeeded in amusing the public for twenty years. In 1769 he composed some of the music for the Shakespeare jubilee at Stratford-on-Avon. His patriotic songs were very popular, and his sea-songs, amongst which are Tom Bowling, Poor Jack, and The Trim-built Wherry, are still favourites in the British navy. He also wrote a History of the Stage, and the novels The Devil and Hannah Hewitt.—His son, Charles Dibdin, composed and wrote many small pieces and occasional songs.—Another son, Thomas, early displayed the same dramatic tastes as his father, was connected with various theatres, and wrote a great many songs and a number of dramas.
Dibdin, Thomas Frognall, an English bibliographer, born in 1776, died in 1847, was the son of the elder brother of Charles Dibdin the celebrated naval song-writer. After studying law and practising as a provincial counsel, he took orders and became a popular preacher in London. Here his bibliographical tastes developed themselves, and the Roxburghe Club being established in 1812, he became its first vice-president. Among his numerous writings may be noted: Bibliomania, Bibliographical Decameron, Typographical Antiquities of Great Britain, and The Library Companion.
Dibranchia´ta. See Cephalopoda.
Dice, cubical pieces of bone or ivory, marked with dots on each of their six faces, from one to six, according to the number of faces. They are shaken in a small box and then thrown on the table. Dice are often loaded or falsified in some way so as to make the high or the low sides turn down. The origin of dice is ascribed to Palamedes of Greece (1244 B.C.), although Herodotus attributes the invention of knuckle-bones and of dice to the Lydians. Dice were well known amongst the ancient Egyptians and Greeks, and are still very popular in Japan, China, India, and other Asiatic countries.
Dicen´tra. See Diëlytra.
Dichasium. See Cyme.
Dichlamydeous (dī-klam-id´i-us), in botany, said of plants that have both calyx and corolla.
Dichogamy, in flowers, the condition in which anthers and stigmas ripen at different times, whereby self-pollination is effectually prevented. If the anthers ripen first, as in borage, columbine, crocus, Caryophyllaceæ, Compositæ, Labiatæ, &c., the flower is said to be protandrous; protogynous flowers, with stigmas ripening and withering before the pollen is shed, occur, e.g. in Christmas rose, Colchicum,