Chapter III: Part 3

DIATOMACEAE. For the knowledge we possess of these beautiful plants, so minute as to be undiscernible by our unaided vision, we are indebted to the assistance of the microscope. It was not till towards the close of the 18th century that the first known forms of this group were discovered by O. F. Muller. And so slow was the process of discovery in this field of scientific research that in the course of half a century, when Agardh published his _Systema algarum_ in 1824, only forty-nine species included under eight genera had been described. Since that time, however, with modern microscopes and microscopic methods, eminent botanists in all parts of the civilized world have studied these minute plants, with the result that the number of known genera and species has been greatly increased. Over 10,000 species of diatoms have been described, and about 1200 species and numerous varieties occur in the fresh waters and on the coasts of Great Britain and Ireland. Rabenhorst, in the index to his _Flora Europaea algarum_ (1864) enumerated about 4000 forms which had up to that time been discovered throughout the continent of Europe.

The diatoms are more commonly known among systematic botanists as the Bacillarieae, particularly on the continent of Europe, and although such an immense number of very diverse forms are included in it, the group as a whole exhibits a remarkable uniformity of structure. The Bacillarieae is one of the large groups of Algae, placed by some in close proximity to the Conjugatae and by others as an order of the Brown Algae (or Phaeophyceae), but their characters are so distinctive and their structure is so uniform as to warrant the separation of the diatoms as a distinct class. The affinities of the group are doubtful.

The diatoms exhibit great variety of form. While some species are circular and more or less disk-shaped, others are oval in outline. Some are linear, as _Synedra Ulna_ (fig. 2), others more or less crescentic; others again are cuneate, as _Podosphenia Lyngbyii_ (fig. 3); some few have a sigmoid outline, as _Pleurosigma balticum_ (fig. 4); but the prevailing forms are naviculoid, as in the large family Naviculaceae, of which the genus _Navicula_ embraces upwards of 1000 species. They vary also in their modes of growth,--some being free-floating, others attached to foreign bodies by simple or branched gelatinous stalks, which in some species are short and thick, while in others they are long and slender. In some genera the forms are simple, while in others the frustules are connected together in ribbon-like filaments, or form, as in other cases, zigzag chains. In some genera the individuals are naked, while in many others they are enclosed in a more or less definite gelatinous investment. The conditions necessary to their growth are moisture and light. Wherever these circumstances coexist, diatomaceous forms will almost invariably be found. They occur mixed with other organisms on the surface of moist rocks; in streamlets and pools, they form a brownish stratum on the surface of the mud, or cover the stems and leaves of water plants or floating twigs with a furry investment. Marine forms are usually attached to various sea-weeds, and many are found in the stomachs of molluscs, holothurians, ascidians and other denizens of the ocean. The fresh-water forms are specifically distinct from those incidental to salt or brackish water,--fresh-water species, however, are sometimes carried some distance into the sea by the force of the current, and in tidal rivers marine forms are carried up by the force of the tide. Some notion may be formed of the extreme minuteness of these forms from the fact that one the length of which is 1/200th of an inch may be considered as beyond the medium size. Some few, indeed, are much larger, but by far the greater proportion are of very much smaller dimensions.

Diatoms are unicellular plants distinguished from kindred forms by the fact of having their soft vegetative part covered by a siliceous case. Each individual is known as a frustule, and the cell-wall consists of two similar valves nearly parallel to each other, each valve being furnished with a rim (or connecting-band) projecting from it at a right angle.

One of these valves with its rim is slightly smaller than the other, the smaller fitting into the larger pretty much as a pill-box fits into its cover. This peculiarity of structure affords ample scope for the growth of the protoplasmic cell-contents, for as the latter increase in volume the siliceous valves are pushed out, and their corresponding siliceous rims become broader. The connecting-bands although closely fitting their respective valves are distinct from them, and together the two bands form the girdle.

An individual diatom is usually described from two aspects, one in which the surface of the valve is exposed to view--the valve view, and one in which the girdle side is exposed--the girdle view. The valves are thin and transparent, convex on the outside, and generally ornamented with a variety of sculptured markings. These sculptures often present the aspect of striae across the face of the valve, and the best lenses have shown them to consist of a series of small cavities within the siliceous wall of the cell. The valves of some of the marine genera exhibit a beautiful areolated structure due to the presence of larger chambers within the siliceous cell-wall. Many diatoms possess thickenings of the cell-wall, visible in the valve view, in the centre of the valve and at each extremity. These thickenings are known as the nodules, and they are generally connected by a long median line, the raphe, which is a cleft in the siliceous valve, extending at least some part of its length.

The protoplasmic contents of this siliceous box-like unicell are very similar to the contents of many other algal cells. There is a living protoplasmic layer or primordial utricle, connected either by two broad bands or by a number of anastomosing threads with a central mass of protoplasm in which the nucleus is embedded. The greater part of the cavity of the cell is occupied by one or several fluid vacuoles. The characteristic brown colour of diatoms is due to the presence of chromatophores embedded in the lining layer of protoplasm. In number and form these chromatophores are variable. They contain chlorophyll, but the green colour is masked by the presence of diatomin, a brown pigment which resembles that which occurs in the Brown Algae or Phaeophyceae. The chromatophores contain a variable number of pyrenoids, colourless proteid bodies of a crystalloidal character.

One of the first phenomena which comes under the notice of the observer is the extraordinary power of motion with which the frustules are endowed. Some species move slowly backwards and forwards in pretty much the same line, but in the case of _Bacillaria paradoxa_ the motion is very rapid, the frustules darting through the water in a zigzag course. To account for this motion various theories have been suggested, none of which appear to be altogether satisfactory. There is little doubt that the movements are connected with the raphe, and in some diatoms there is much evidence to prove that they are due to an exudation of mucilage.

_Classification._--The most natural system of classification of the Bacillarieae is the one put forward by Schutt (1896), and since generally followed by systematists. He separates them into two primary divisions, the 'Centricae' and the 'Pennatae.' The former includes all those diatoms which in the valve view possess a radial symmetry around a central point, and which are destitute of a raphe (or a pseudoraphe). The latter includes those which are zygomorphic or otherwise irregular, and in which the valve view is generally boat-shaped or needle-shaped, with the markings arranged in a sagittal manner on each side of a raphe or pseudoraphe.

_Reproduction._--In the Diatomaceae, as well as in the Desmidieae, the ordinary mode of increase is by simple cell-division. The cell-contents within the enclosure of the siliceous case separate into two distinct masses. As these two daughter-masses become more and more developed, the valves of the mother-cell are pushed more and more widely apart. A new siliceous valve is secreted by each of the two masses on the side opposite to the original valve, the new valves being situated within the girdle of the original frustule. When this process has been completed the girdle of the mother frustule gives way, and two distinct frustules are formed, the siliceous valves in each of these new frustules being one of the valves of the mother-cell, and a newly formed valve similar and more or less parallel to it.

During the life of the plant this process of self-division is continued with an almost incredible rapidity. On this subject the observation of Professor William Smith, writing in 1853, is worthy of special notice:--"I have been unable to ascertain the time occupied in a single act of self-division, but supposing it to be completed in twenty-four hours we should have, as the progeny of a single frustule, the amazing number of 1,000,000,000 in a single month, a circumstance which will in some degree explain the sudden, or at least rapid, appearance of these organisms in localities where they were a short time previously either unrecognized or sparingly diffused" (_British Diatomaceae_, vol. i. p. 25).

A. _Navicula limosa._
B. _Achnanthes flexella._
C. _Navicula Amphisbaena._
D. _Navicula viridis._]

Individual diatoms when once produced by cell-division are incapable of any increase in size owing to the rigidity of their siliceous cell-walls, and since the new valves are always formed _within_ the girdle of the old ones, it would follow that every succeeding generation is reduced in size by the thickness of the girdle. In some diatoms, however, this is not strictly true as daughter-cells are sometimes produced of larger size than the parent-cells. Thus, the reduction in size of the individuals is not always proportionate to the number of cell-divisions.

On the diminution in size having reached a limit in any species, the maximum size is regained by the formation of an auxospore. There are five known methods of reproduction by auxospores, but it is unnecessary here to enter into details of these methods. Suffice it to say that a normal auxospore is produced by the conjugation of two parent-cells, its distinguishing feature being a rejuvenescence accompanied by a marked increase in size. These auxospores formed without conjugation are parthenogenetic.

_Mode of Preparation._--The Diatomaceae are usually gathered in small bottles, and special care should be taken to collect them as free as possible from extraneous matter. A small portion having been examined under the microscope, should the gathering be thought worthy of preservation, some of the material is boiled in acid for the purpose of cleaning it. The acids usually employed are hydrochloric, nitric or sulphuric, according as circumstances require. When the operator considers that by this process all foreign matter has been eliminated, the residuum is put into a precipitating jar of a conical shape, broader at the bottom than at the top, and covered to the brim with filtered or distilled water. When the diatoms have settled in the bottom of the jar, the supernatant fluid is carefully removed by a syringe or some similar instrument, so that the sediment be not disturbed. The jar is again filled with water, and the process repeated till the acid has been completely removed. It is desirable afterwards to boil the sediment for a short time with supercarbonate of soda, the alkali being removed in the same manner as the acid. A small portion may then be placed with a pipette upon a slip of glass, and, when the moisture has been thoroughly evaporated, the film that remains should be covered with dilute Canada balsam, and, a thin glass cover having been gently laid over the balsam, the preparation should be laid aside for a short time to harden, and then is ready for observation.

_General Remarks._--Diatoms are most abundant in cold latitudes, having a general preference for cold water. In the pelagic waters of lakes and of the oceans they are often very abundant, and in the cold waters of the Arctic and Antarctic Oceans they exist in prodigious numbers. They thus form a large proportion of both the marine and the fresh-water plankton.

Large numbers of fossil diatoms are known. Not only are these minute plants assisting at the present time in the accumulation of oceanic and lake deposits, but in former ages they have been sufficiently active to give rise to considerable deposits of diatomaceous earths. When the plant has fulfilled its natural course the siliceous covering sinks to the bottom of the water in which it had lived, and there forms part of the sediment. When in the process of ages, as it has often happened, the accumulated sediment has been hardened into solid rock, the siliceous frustules of the diatoms remain unaltered, and, if the rock be disintegrated by natural or artificial means, may be removed from the enveloping matrix and subjected to examination under the microscope. The forms found may from their character help in some degree to illustrate the conditions under which the stratum of rock had been originally deposited. These earths are generally of a white or grey colour. Some of them are hard, but most are soft and friable. Many of them are of economic importance, being used as polishing powders ("Tripoli"), as absorbents for nitroglycerin in the manufacture of dynamite ("Kieselguhr"), as a dentifrice, and more recently they have been used to a large extent in the manufacture of non-conducting and sound-proof materials. Most of these diatomaceous earths are associated with rocks of Tertiary formations, although it is generally regarded that the earliest appearance of diatoms is in the Upper Cretaceous (chalk).

Vast deposits of Diatomaceous earths have been discovered in various parts of the world,--some the deposit of fresh, others of salt water. Of these deposits the most remarkable for extent, as well as for the number and beauty of the species contained in it, is that of Richmond, in Virginia, one of the United States of America. It extends for many miles, and is in some places at least 40 ft. deep. It is a remarkable fact that though the generations of a diatom in the space of a few months far exceed in number the generation of man during the period usually assigned to the existence of the race, the fossil genera and species are in most respects to the most minute details identical with the numerous living representatives of their class. (E. O'M.; G. S. W.*)

DIAULOS (from Gr. [Greek: di-], double, and [Greek: aulos], pipe), in architecture, the peristyle round the great court of the palaestra, described by Vitruvius (v. II), which measured two stadia (1200 ft.) in length; on the south side this peristyle had two rows of columns, so that in stormy weather the rain might not be driven into the inner part. The word was also used in ancient Greece for a foot-race of twice the usual length.

DIAVOLO, FRA (1771-1806), the popular name given to a famous Italian brigand associated with the political revolutions of southern Italy at the time of the French invasion. His real name was Michele Pezza, and he was born of low parentage at Itri; he had committed many murders and robberies in the Terra di Lavoro, but by good luck combined with audacity he always escaped capture, whence his name of Fra Diavolo, popular superstition having invested him with the characters of a monk and a demon, and it seems that at one time he actually was a monk. When the kingdom of Naples was overrun by the French and the Parthenopaean Republic established (1799), Cardinal Ruffo, acting on behalf of the Bourbon king Ferdinand IV., who had fled to Sicily, undertook the reconquest of the country, and for this purpose he raised bands of peasants, gaol-birds, brigands, &c., under the name of Sanfedisti or _bande della Santa Fede_ ("bands of the Holy Faith"). Fra Diavolo was made leader of one of them, and waged untiring war against the French troops, cutting off isolated detachments and murdering stragglers and couriers. Owing to his unrivalled knowledge of the country, he succeeded in interrupting the enemy's communications between Rome and Naples. But although, like his fellow-brigands under Ruffo, he styled himself "the faithful servant and subject of His Sicilian Majesty," wore a military uniform and held military rank, and was even created duke of Cassano, his atrocities were worthy of a bandit chief. On one occasion he threw some of his prisoners, men, women and children, over a precipice, and on another he had a party of seventy shot. His excesses while at Albano were such that the Neapolitan general Naselli had him arrested and imprisoned in the castle of St Angelo, but he was liberated soon after. When Joseph Bonaparte was made king of Naples, extraordinary tribunals were established to suppress brigandage, and a price was put on Fra Diavolo's head. After spreading terror through Calabria, he crossed over to Sicily, where he concerted further attacks on the French. He returned to the mainland at the head of 200 convicts, and committed further excesses in the Terra di Lavoro; but the French troops were everywhere on the alert to capture him and he had to take refuge in the woods of Lenola. For two months he evaded his pursuers, but at length, hungry and ill, he went in disguise to the village of Baronissi, where he was recognized and arrested, tried by an extraordinary tribunal, condemned to death and shot. In his last moments he cursed both the Bourbons and Admiral Sir Sidney Smith for having induced him to engage in this reckless adventure (1806). Although his cruelty was abominable, he was not altogether without generosity, and by his courage and audacity he acquired a certain romantic popularity. His name has gained a world-wide celebrity as the title of a famous opera by Auber.

The best known account of Fra Diavolo is in Pietro Colletta's _Storia
del reame di Napoli_ (2nd ed., Florence, 1848); B. Amante's _Fra
Diavolo e il suo tempo_ (Florence, 1904) is an attempted
rehabilitation; but A. Luzio, whose account in _Profili e bozzetti
storici_ (Milan, 1906) gives the latest information on the subject,
has demolished Amante's arguments. (L. V.*)

DIAZ, NARCISSE VIRGILIO (1808-1876), French painter, was born in Bordeaux of Spanish parents, on the 25th of August 1808. At first a figure-painter who indulged in strong colour, in his later life Diaz became a painter of the forest and a "tone artist" of the first order. He spent much time at Barbizon; and although he is the least exalted of the half-dozen great artists who are usually grouped round that name, he sometimes produced works of the highest quality. At the age of ten Diaz became an orphan, and misfortune dogged his earlier years. His foot was bitten by a reptile in Meudon wood, near Sevres, where he had been taken to live with some friends of his mother. The bite was badly dressed, and ultimately it cost him his leg. Afterwards his wooden stump became famous. At fifteen he entered the studios at Sevres, where the decoration of porcelain occupied him; but tiring of the restraint of fixed hours, he took to painting Eastern figures dressed in richly coloured garments. Turks and Oriental scenes attracted him, and many brilliant gems remain of this period. About 1831 Diaz encountered Theodore Rousseau, for whom he entertained a great veneration, although Rousseau was four years his junior; but it was not until ten years later that the remarkable incident took place of Rousseau teaching Diaz to paint trees. At Fontainebleau Diaz found Rousseau painting his wonderful forest pictures, and determined to paint in the same way if possible. Rousseau, then in poor health, worried at home, and embittered against the world, was difficult to approach. Diaz followed him surreptitiously to the forest,--wooden leg not hindering,--and he dodged round after the painter, trying to observe his method of work. After a time Diaz found a way to become friendly with Rousseau, and revealed his anxiety to understand his painting. Rousseau was touched with the passionate words of admiration, and finally taught Diaz all he knew. Diaz exhibited many pictures at the Paris Salon, and was decorated in 1851. During the Franco-German War he went to Brussels. After 1871 he became fashionable, his works gradually rose in the estimation of collectors, and he worked constantly and successfully. In 1876 he caught cold at his son's grave, and on the 18th of November of that year he died at Mentone, whither he had gone to recruit his health. Diaz's finest pictures are his forest scenes and storms, and it is on these, and not on his pretty figures, that his fame is likely to rest. There are several fairly good examples of the master in the Louvre, and three small figure pictures in the Wallace collection, Hertford House. Perhaps the most notable of Diaz's works are "La Fee aux Perles" (1857), in the Louvre; "Sunset in the Forest" (1868); "The Storm," and "The Forest of Fontainebleau" (1870) at Leeds. Diaz had no well-known pupils, but Leon Richet followed markedly his methods of tree-painting, and J. F. Millet at one period painted small figures in avowed imitation of Diaz's then popular subjects.

See A. Hustin, _Les Artistes celebres: Diaz_ (Paris); D. Croal
Thomson, _The Barbizon School of Painters_ (London, 1890); J. W.
Mollett, _Diaz_ (London, 1890); J. Claretie, _Peintres et sculpteurs
contemporains: Diaz_ (Paris, 1882); Albert Wolff, _La Capitale de
l'art: Narcisse Diaz_ (Paris, 1886); Ph. Burty, _Maitres et
petit-maitres: N. Diaz_ (Paris, 1877). (D.C.T.)

DIAZ, PORFIRIO (1830- ), president of the republic of Mexico (q.v.), was born in the southern state of Oaxaca, on the 15th of September 1830. His father was an innkeeper in the little capital of that province, and died three years after the birth of Porfirio, leaving a family of seven children. The boy, who had Indian blood in his veins, was educated for the Catholic Church, a body having immense influence in the country at that time and ordering and controlling revolutions by the strength of their filled coffers. Arrived at the age of sixteen Porfirio Diaz threw off the authority of the priests. Fired with enthusiasm by stories told by the revolutionary soldiers continually passing through Oaxaca, and hearing about the war with the United States, a year later he determined to set out for Mexico city and join the National Guard. There being no trains, and he being too poor to ride, he walked the greater part of the 250 m., but arrived there too late, as the treaty of Guadalupe-Hidalgo (1848) had been already signed, and Texas finally ceded to the United States. Thus his entering the army was for the time defeated. Thereupon he returned to his native town and began studying law. He took pupils in order to pay his own fees at the Law Institute, and help his mother. At this time he came under the notice and influence of Don Marcos Perez and Benito Juarez, the first a judge, the second a governor of the state of Oaxaca, and soon to become famous as the deliverer of Mexico from the priesthood (War of Reform). Diaz continued in his native town until 1854, when, refusing to vote for the dictator, Santa Anna, he was stung by a taunt of cowardice, and hastily pushing his way to the voting place, he recorded his vote in favour of Alvarez and the revolutionists. Orders were given for his arrest, but seizing a rifle and mounting a horse he placed himself at the head of a few revolting peasants, and from that moment became one of the leading spirits in that long struggle for reform, known as the War of Reform, which, under the leadership of Juarez, followed the overthrow of Santa Anna. Promotion succeeded promotion, as Diaz led his troops from victory to victory, amid great privations and difficulties. He was made captain (1856), lieutenant-colonel and colonel (1859), brigadier-general (1861), and general of division for the army (1863). Closely following on civil war, political strife, open rebellion and the great War of Reform, came the French invasion of 1862, and the landing of the emperor Maximilian in 1864. From the moment the French disclosed their intentions of settling in Mexico in 1862, Diaz took a prominent part against the foreign invasion. He was twice seriously wounded, imprisoned on three different occasions, had two hairbreadth escapes, and took part in many daring engagements. So important a personage did he become that both Marshal Bazaine and the emperor Maximilian made overtures to him. At the time of Maximilian's death (with which Diaz personally had nothing to do) he was carrying on the siege of Mexico city, which ended in the surrender of the town two days after the emperor was shot at Queretaro between his two leading generals. Diaz at once set to work to pay up arrears due to his soldiers, proclaimed death as the penalty of plunder and theft, and in the few weeks that followed showed his great administrative powers, the officers as well as the rank and file receiving arrears of pay. On the very day that he occupied Mexico city, the great commander of the army of the east, to everyone's surprise, sent in his resignation. He was, indeed, appointed to the command of the second division of the army by President Juarez in his military reorganization, but Diaz, seeing men who had given great and loyal service to the state dismissed from their positions in the government, and disgusted at this course, retired to the little city of Oaxaca; there he lived, helping in the reorganization of the army but taking no active part in the government until 1871.

On Juarez' death Lerdo succeeded as president, in 1872. His term of office again brought discord, and when it was known that he was attempting to be re-elected in 1876, the storm broke. Diaz came from retirement, took up the leadership against Lerdo, and after desperate struggles and a daring escape finally made a triumphal entry into Mexico city on the 24th of November 1876, as provisional president, quickly followed by the full presidentship. His term of office marks a prominent change in the history of Mexico; from that date he at once forged ahead with financial and political reform, the scrupulous settlement of all national debts, the welding together of the peoples and tribes (there are 150 different Indian tribes) of his country, the establishment of railroads and telegraphs, and all this in a land which had been upheaved for a century with revolutions and bloodshed, and which had had fifty-two dictators, presidents and rulers in fifty-nine years. In 1880 Diaz was succeeded by Gonzalez, the former minister of war, for four years (owing to the limit of the presidential office), but in 1884 he was unanimously re-elected. The government having set aside the above-mentioned limitation, Diaz was continually re-elected to the presidency. He married twice and had a son and two daughters. His gifted second wife (Carmelita), very popular in Mexico, was many years younger than himself. King Edward VII. made him an honorary grand commander of the Bath in June 1906, in recognition of his wonderful administration as perpetual president for over a quarter of a century.

See also Mrs Alec Tweedie, _Porfirio Diaz, Seven Times President of
Mexico_ (1906), and _Mexico as I saw it_ (1901); Dr Noll, _From Empire
to Republic_ (1890); Lieut. Seaton Schroeder, _Fall of Maximilian's
Empire_ (New York, 1887); R. de Z. Enriquez, _P. Diaz_ (1908); and an
article by Percy Martin in _Quarterly Review_ for October 1909.
(E. A. T.)

DIAZ DE NOVAES, BARTHOLOMEU (fl. 1481-1500), Portuguese explorer, discoverer of the Cape of Good Hope, was probably a kinsman of Joao Diaz, one of the first Portuguese to round Cape Bojador (1434), and of Diniz Diaz, the discoverer of Cape Verde (1445). In 1478 a Bartholomeu Diaz, probably identical with the discoverer, was exempted from certain customary payments on ivory brought from the Guinea coast. In 1481 he commanded one of the vessels sent by King John II. under Diogo d'Azambuja to the Gold Coast. In 1486 he seems to have been a cavalier of the king's household, and superintendent of the royal warehouses; on the 10th of October in this year he received an annuity of 6000 reis from King John for "services to come"; and some time after this (probably about July or August 1487, rather than July 1486, the traditional date) he left Lisbon with three ships to carry on the work of African exploration so greatly advanced by Diogo Cao (1482-1486). Passing Cao's farthest point near Cape Cross (in the modern German South-west Africa and) in 21 deg. 50' S., he erected a pillar on what is now known as Diaz Point, south of Angra Pequena or Luderitz Bay, in 26 deg. 38' S.; of this fragments still exist. From this point (according to De Barros) Diaz ran thirteen days southwards before strong winds, which freshened to dangerous stormy weather, in a comparatively high southern latitude, considerably south of the Cape. When the storm subsided the Portuguese stood east; and failing, after several days' search, to find land, turned north, and so struck the south coast of Cape Colony at Mossel Bay (Diaz' Bahia dos Vaqueiros), half way between the Cape of Good Hope and Port Elizabeth (February 3, 1488). Thence they coasted eastward, passing Algoa Bay (Diaz' Bahia da Roca), erecting pillars (or perhaps wooden crosses), it is said, on one of the islands in this bay and at or near Cape Padrone farther east; of these no traces remain. The officers and men now began to insist on return, and Diaz could only persuade them to go as far as the estuary of the Great Fish River (Diaz' Rio do Iffante, so named from his colleague, Captain Joao Iffante). Here, however, half way between Port Elizabeth and East London (and indeed from Cape Padrone), the north-easterly trend of the coast became unmistakable; the way round Africa had been laid open. On his return Diaz perhaps named Cape Agulhas after St Brandan; while on the southernmost projection of the modern Cape peninsula, whose remarkable highlands (Table Mountain, &c.) doubtless impressed him as the practical termination of the continent, he bestowed, says De Barros, the name of Cape of Storms (_Cabo Tormentoso_) in memory of the storms he had experienced in these far southern waters; this name (in the ordinary tradition) was changed by King John to that of Good Hope (_Cabo da Boa Esperanca_). Some excellent authorities, however, make Diaz himself give the Cape its present name. Hard by this "so many ages unknown promontory" the explorer probably erected his last pillar. After touching at the Ilha do Principe (Prince's Island, south-west of the Cameroons) as well as at the Gold Coast, he appeared at Lisbon in December 1488. He had discovered 1260 m. of hitherto unknown coast; and his voyage, taken with the letters soon afterwards received from Pero de Covilhao (who by way of Cairo and Aden had reached Malabar on one side and the "Zanzibar coast" on the other as far south as Sofala, in 1487-1488) was rightly considered to have solved the question of an ocean route round Afr ica to the Indies and other lands of South and East Asia.

No record has yet been found of any adequate reward for Diaz: on the contrary, when the great Indian expedition was being prepared (for Vasco da Gama's future leadership) Bartolomeu only superintended the building and outfit of the ships; when the fleet sailed in 1497, he only accompanied da Gama to the Cape Verde Islands, and after this was ordered to El Mina on the Gold Coast. On Cabral's voyage of 1500 he was indeed permitted to take part in the discovery of Brazil (April 22), and thence should have helped to guide the fleet to India; but he perished in a great storm off his own Cabo Tormentoso. Like Moses, as Galvano says, he was allowed to see the Promised Land, but not to enter in.

See Joao de Barros, _Asia_, Dec. I. bk. iii. ch. 4; Duarte Pacheco
Pereira, _Esmeraldo de situ orbis_, esp. pp. 15, 90, 92, 94 and
Raphael Bastos's introduction to the edition of 1892 (Pacheco met
Diaz, returning from his great voyage, at the Ilha do Principe); a
marginal note, probably by Christopher Columbus himself, on fol. 13 of
a copy of Pierre d'Ailly's _Imago mundi_, now in the Colombina at
Seville (the writer of this note fixes Diaz's return to Lisbon,
December 1488, and says he was present at Diaz's interview with the
king of Portugal, when the explorer described his voyage and showed
his route upon the chart he had kept); a similar but briefer note in a
copy of Pope Pius II.'s _Historia rerum ubique gestarum_, from the
same 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, crystallizes in long silky needles. The sulphate and chloride are similar, but they are not quite so unstable as the nitrate. The bromide may be prepared by the addition of bromine to an ethereal solution of diazo-amino-benzene (tribromaniline remaining in solution). By the addition of potassium bromide and bromine water to diazonium salts they are converted into a _perbromide_, e.g. C6H5N2Br3, which crystallizes in yellow plates.

The diazonium salts are characterized by their great reactivity and
consequently are important reagents in synthetical processes, since by
their agency the amino group in a primary amine may be exchanged for
other elements or radicals. The chief reactions are as follows:--

1. _Replacement of -NH2 by -OH_:--The amine is diazotized and the
aqueous solution of the diazonium salt is heated, nitrogen being
eliminated and a phenol formed.

2. _Replacement of -NH2 by halogens and by the -CN and -CNO
groups_:--The diazonium salt is warmed with an acid solution of the
corresponding cuprous salt (T. Sandmeyer, _Ber._, 1884, 17, p. 2650),
or with copper powder (L. Gattermann, Ber., 1890, 23, p. 1218; 1892,
25, p. 1074). In the case of iodine, the substitution is effected by
adding a warm solution of potassium iodide to the diazonium solution,
no copper or cuprous salt being necessary; whilst for the production
of nitriles a solution of potassium cuprous cyanide is used. This
reaction (the so-called "Sandmeyer" reaction) has been investigated by
A. Hantzsch and J. W. Blagden (_Ber._, 1900, 33, p. 2544), who
consider that three simultaneous reactions occur, namely, the
formation of labile double salts which decompose in such a fashion
that the radical attached to the copper atom wanders to the aromatic
nucleus; a catalytic action, in which nitrogen is eliminated and the
acid radical attaches itself to the aromatic nucleus; and finally, the
formation of azo compounds.

3. _Replacement of -NH2 by -NO2_:--A well-cooled concentrated solution
of potassium mercuric nitrate is added to a cooled solution of benzene
diazonium nitrate, when the crystalline salt 2C6H5N2.NO3, Hg(NO2)2 is
precipitated. On warming this with copper powder, it gives a
quantitative yield of nitrobenzene (A. Hantzsch, _Ber._, 1900, 33, p.
2551).

4. _Replacement of -NH2 by hydrogen_:--This exchange is brought about,
in some cases, by boiling the diazonium salt with alcohol; but I.
Remsen and his pupils (_Amer. Chem. Journ._, 1888, 9, pp. 389 et seq.)
have shown that the main product of this reaction is usually a
phenolic ether. This reaction has also been investigated by A.
Hantzsch and E. Jochem (_Ber._, 1901, 34, p. 3337), who arrived at the
conclusion that the normal decomposition of diazonium salts by
alcohols results in the formation of phenolic ethers, but that an
increase in the molecular weight of the alcohol, or the accumulation
of negative groups in the aromatic nucleus, diminishes the yield of
the ether and increases the amount of the hydrocarbon formed. The
replacement is more readily brought about by the use of sodium
stannite (P. Friedlander, _Ber._, 1889, 22, p. 587), or by the use of
a concentrated solution of hypophosphorous acid (J. Mai, _Ber._, 1902,
35, p. 162). A. Hantzsch (_Ber._, 1896, 29, p. 947; 1898, 31, p. 1253)
has shown that the chlor- and brom- diazoniumthiocyanates, when
dissolved in alcohol containing a trace of hydrochloric acid, become
converted into the isomeric thiocyanbenzene diazonium chlorides and
bromides. This change only occurs when the halogen atom is in the
ortho- or para- position to the -N2- group.

_Metallic Diazo Derivatives._--Benzene diazonium chloride is
decomposed by silver oxide in aqueous solution, with the formation of
_benzene diazonium hydroxide_, C6H5.N(OH):.N. This hydroxide, although
possessing powerful basic properties, is unstable in the presence of
alkalis and neutralizes them, being converted first into the isomeric
benzene-diazotic acid, the potassium salt of which is obtained when
the diazonium chloride is added to an excess of cold concentrated
potash (A. Hantzsch and W. B. Davidson, _Ber._, 1898, 31, p. 1612).
_Potassium benzene diazotate_, C6H5N2.OK, crystallizes in colourless
silky needles. The free acid is not known; by the addition of the
potassium salt to 50% acetic acid at -20 deg. C., the acid anhydride,
_benzene diazo oxide_, (C6H5N2)2O, is obtained as a very unstable,
yellow, insoluble compound, exploding spontaneously at 0 deg. C. Strong
acids convert it into a diazonium salt, and potash converts it into
the diazotate. On the constitution, of these anhydrides see E.
Bamberger, _Ber._, 1896, 29, p. 446, and A. Hantzsch, _Ber._, 1896,
29, p. 1067; 1898, 31, p. 636. By the addition of the diazonium salts
to a hot concentrated solution of a caustic alkali, C. Schraube and C.
Schmidt (_Ber._, 1894, 27, p. 520) obtained an isomer of potassium
benzene diazotate. These _iso-_diazotates are formed much more readily
when the aromatic nucleus in the diazonium salt contains negative
radicals. _Potassium benzene iso-diazotate_ resembles the normal salt,
but is more stable, and is more highly ionized. Carbon dioxide
converts it into _phenyl nitrosamine_, C6H5NH.NO (A. Hantzsch). The
potassium salt of the iso-diazo hydroxide yields on methylation a
nitrogen ether, R.N(CH3).NO, whilst the silver salt yields an oxygen
ether, R.N:N.OCH3. These results point to the conclusion that the
iso-diazo hydroxide is a tautomeric substance. The same oxygen ether
is formed by the methylation of the silver salt of the normal diazo
hydroxide; this points to the conclusion that the isomeric hydroxides,
corresponding with the silver derivatives, have the same structural
formulae, namely, R.N:N.OH. These oxygen ethers contain the grouping
-N:N-, since they couple very readily with the phenols in alkaline
solution to form azo compounds (q.v.) (E. Bamberger, _Ber._, 1895, 28,
p. 225); they are also explosive.

By oxidizing potassium benzene iso-diazotate with alkaline potassium
ferricyanide, E. Bamberger (_Ber._, 1894, 27, p. 914) obtained the
_diazoic acids_, R.NH.NO2, substances which he had previously prepared
by similarly oxidizing the diazonium salts, by dehydrating the
nitrates of primary amines with acetic anhydride, and by the action of
nitric anhydride on the primary amines. Concentrated acids convert
them into the isomeric nitro-amines, the -NO2 group going into the
nucleus in the ortho- or para- position to the amine nitrogen; this
appears to indicate that the compounds are nitramines. They behave,
however, as tautomeric substances, since their alkali salts on
methylation give nitrogen ethers, whilst their silver salts yield
oxygen ethers:

/--> potassium salt --> R.N(CH3).NO2 nitramine.
R.NH.NO2<
\--> silver salt --> R.N:N.O.OCH3 diazoate.

_Phenyl nitramine_, C6H5NH.NO2, is a colourless crystalline solid,
which melts at 46 deg. C. Sodium amalgam in alkaline solution reduces it
to phenylhydrazine.

_Constitution of the Diazo Compounds._--P. Griess (_Ann._, 1866, 137,
p. 39) considered that the diazo compounds were formed by the addition
of complex groupings of the type C6H4N2- to the inorganic acids;
whilst A. Kekule (_Zeit. f. Chemie_, 1866, 2, p. 308), on account of
their ready condensation to form azo compounds and their easy
reduction to hydrazines, assumed that they were substances of the type
R.N:N.Cl. The constitution of the diazonium group -N2.X, may be
inferred from the following facts:--The group C6H5N2- behaves in many
respects similarly to an alkali metal, and even more so to the
ammonium group, since it is capable of forming colourless neutral
salts with mineral acids, which in dilute aqueous solution are
strongly ionized, but do not show any trace of hydrolytic dissociation
(A. Hantzsch, _Ber._, 1895, 28, p. 1734). Again, the diazonium
chlorides combine with platinic chloride to form difficultly soluble
double platinum salts, such as (C6H5N2Cl)2.PtCl4; similar gold salts,
C6H5N2Cl.AuCl3, are known. Determinations of the electrical
conductivity of the diazonium chloride and nitrate also show that the
diazonium radical is strictly comparable with other quaternary
ammonium ions. For these reasons, one must assume the existence of
pentavalent nitrogen in the diazonium salts, in order to account for
their basic properties.

The constitution of the isomeric diazo hydroxides has given rise to
much discussion. E. Bamberger (_Ber._, 1895, 28, pp. 444 et seq.) and
C. W. Blomstrand (_Journ. prakt. Chem._, 1896, 53, pp. 169 et seq.)
hold that the compounds are structurally different, the normal
diazo-hydroxide being a diazonium derivative of the type
R.N([3:]N).OH. The recent work of A. Hantzsch and his pupils seems to
invalidate this view (_Ber._, 1894, 27, pp. 1702 et seq.; see also A.
Hantzsch, _Die Diazoverbindungen_). According to Hantzsch the isomeric
diazo hydroxides are structurally identical, and the differences in
behaviour are due to stereo-chemical relations, the isomerism being
comparable with that of the oximes (q.v.). On such a hypothesis, the
relatively unstable normal diazo hydroxides would be the
_syn-_compounds, since here the nitrogen atoms would be more easily
eliminated, whilst the stable iso-diazo derivatives would be the
_anti-_compounds, thus:

R . N R . N
.. ..
HO . N N . OH
Normal hydroxide Iso hydroxide
(Syn-compound) (Anti-compound)

In support of this theory, Hantzsch has succeeded in isolating a
series of syn- and anti-diazo-cyanides and -sulphonates (_Ber._, 1895,
28, p. 666; 1900, 33, p. 2161; 1901, 34, p. 4166). By diazotizing
para-chloraniline and adding a cold solution of potassium cyanide, a
salt (melting at 29 deg. C.) is obtained, which readily loses
nitrogen, and forms para-chlorbenzonitrile on the addition of copper
powder. By dissolving this diazocyanide in alcohol and reprecipitating
it by water, it is converted into the isomeric diazocyanide (melting
at 105-106 deg. C.), which does not yield para-chlorbenzonitrile when
treated with copper powder. Similar results have been obtained by
using diazotized para-anisidine, a syn- and an anti- compound being
formed, as well as a third isomeric cyanide, obtained by evaporating
para-methoxy-benzenediazonium hydroxide in the presence of an excess
of hydrocyanic acid at ordinary temperatures. This salt is a
colourless crystalline substance of composition
CH3O.C6H4.N2.CN.HCN.2H2O, and has the properties of a metallic salt;
it is very soluble in water and its solution is an electrolyte,
whereas the solutions of the syn-and anti- compounds are not
electrolytes. The isolation of these compounds is a powerful argument
in favour of the Hantzsch hypothesis which requires the existence of
these three different types, whilst the Bamberger-Blomstrand view only
accounts for the formation of two isomeric cyanides, namely, one of
the normal diazonium type and one of the iso-diazocyanide type.

Benzene diazonium hydroxide, although a strong base, reacts with the
alkaline hydroxides to form salts with the evolution of heat, and
generally behaves as a weak acid. On mixing dilute solutions of the
diazonium hydroxide and the alkali together, it is found that the
molecular conductivity of the mixture is much less than the sum of the
two electrical conductivities of the solutions separately, from which
it follows that a portion of the ions present have changed to the
non-ionized condition. This behaviour is explained by considering the
non-ionized part of the diazonium hydroxide to exist in solution in a
hydrated form, the equation of equilibrium being:

C6H5.N. --> C6H5.N.OH
H2O + ... + OH' |
N <-- HO.N.H

On adding the alkaline hydroxide to the solution, this hydrate is
supposed to lose water, yielding the syn-diazo hydroxide, which then
gives rise to a certain amount of the sodium salt (A. Hantzsch,
_Ber._, 1898, 31, p. 1612),

....
C6H5.N.:OH: --> C6H5.N --> C6H5.N
| : : || ||
HO.N.:H : <-- HO.N <-- NaO.N
....

This assumption also shows the relationship of the diazonium
hydroxides to other quaternary ammonium compounds, for most of the
quaternary ammonium hydroxides (except such as have the nitrogen atom
attached to four saturated hydrocarbon radicals) are unstable, and
readily pass over into compounds in which the hydroxyl group is no
longer attached to the amine nitrogen; thus the syn-diazo hydroxides
are to be regarded as pseudo-diazonium derivatives. (A. Hantzsch,
_Ber._, 1899, 32, p. 3109; 1900, 33, p. 278.) It is generally accepted
that the iso-diazo hydroxides possess the oxime structure R.N:N.OH.

Hantzsch explains the characteristic reactions of the diazonium
compounds by the assumption that an addition compound is first formed,
which breaks down with the elimination of the hydride of the acid
radical, and the formation of an unstable syn-diazo compound, which,
in its turn, decomposes with evolution of nitrogen (_Ber._, 1897, 30,
p. 2548; 1898, 31, p. 2053).

R X R X R X
\ | \ / | |
N.:N + | --> N.:N --> | | + HCl --> R.X + N2.
/ | / \ | |
Cl H Cl H N = N

J. Cain (_Jour. Chem. Soc._, 1907, 91, p. 1049) suggested a quinonoid
formula for diazonium salts, which has been combated by Hantzsch
(_Ber._, 1908, 41, pp. 3532 et seq.). G. T. Morgan and F. M. G.
Micklethwaite (_Jour. Chem. Soc._, 1908, 93, p. 617; 1909, 95, p.
1319) have pointed out that the salts may possess a dynamic formula,
Cain's representing the middle stage, thus:

/||\ /||\ /||\
/ || \ / || \ / || \
H / N.Cl \ H H / N.Cl \ H H / N.Cl \ H
|\ || || ---> || || | ---> || || /|
| \ || || <--- || || | <--- || || / |
| \N || || N | || N |
H\\ / H H\\ | / H H \ //H
\\ / \\ | / \ //
\\ / \\| / \ //
H H H

_Diazoamines._--The diazoamines, R.N2.NHR, may be prepared by the
action of the primary and secondary amines on the diazonium salts, or
by the action of nitrous acid on the free primary amine. In the latter
reaction it is assumed that the isodiazohydroxide first formed is
immediately attacked by a second molecule of the amine. They are
yellow crystalline solids, which do not unite with acids. Nitrous acid
converts them, in acid solution, into diazonium salts.

C6H5N2.NHC6H5 + 2HCl + HNO2 = 2C6H5N2Cl + 2H2O.

They are readily converted into the isomeric aminoazo compounds,
either by standing in alcoholic solution, or by warming with a mixture
of the parent base and its hydrochloride; the diazo group preferably
going into the para-position to the amino group. When the
para-position is occupied, the diazo group takes the ortho-position.
H. Goldschmidt and R. U. Reinders (_Ber._, 1896, 29, p. 1369, 1899)
have shown that the transformation is a monomolecular reaction, the
velocity of transformation in moderately dilute solution being
independent of the concentration, but proportional to the amount of
the catalyst present (amine hydrochloride) and to the temperature. It
has also been shown that when different salts of the amine are used,
their catalytic influence varies in amount and is almost proportional
to their degree of ionization in aqueous solution. Diazoaminobenzene,
C6H5N2.NHC6H5, crystallizes in golden yellow laminae, which melt at
96 deg. C. and explode at a slightly higher temperature. It is readily
soluble in alcohol, ether and benzene. Concentrated hydrochloric acid
converts it into chlorbenzene, aniline and nitrogen. Zinc dust and
alcoholic acetic acid reduce it to aniline and phenylhydrazine.

_Diazoimino compounds_, R.N3, may be regarded as derivatives of
azoimide (q.v.); they are formed by the action of ammonia on the
diazoperbromides, or by the action of hydroxylamine on the diazonium
sulphates (J. Mai, _Ber._, 1892, 25, p. 372; T. Curtius, _Ber._, 1893,
26, p. 1271). Diazobenzeneimide, C6H5N3, is a yellowish oil of
stupefying odour. It boils at 59 deg. C. (12 mm.), and explodes when
heated. Concentrated hydrochloric acid decomposes it with formation of
chloranilines and elimination of nitrogen, whilst on boiling with
sulphuric acid it is converted into aminophenols.

_Aliphatic Diazo Compounds._--The esters of the aliphatic amino acids
may be diazotized in a manner similar to the primary aromatic amines,
a fact discovered by T. Curtius (_Ber._, 1833, 16, p. 2230). The first
aliphatic diazo compound to be isolated was _diazoacetic ester_,
CH.N2.CO2C2H5, which is prepared by the action of potassium nitrite on
the ethyl ester of glycocoll hydrochloride, HCl.NH2.CH2.CO2C2H5 + KNO2
= CHN2.CO2C2H5 + KCl + 2H2O. It is a yellowish oil which melts at -24
deg. C.; it boils at 143-144 deg. C., but cannot be distilled safely
as it decomposes violently, giving nitrogen and ethyl fumarate. It
explodes in contact with concentrated sulphuric acid. On reduction it
yields ammonia and glycocoll (aminoacetic acid). When heated with
water it forms ethyl hydroxy-acetate; with alcohol it yields ethyl
ethoxyacetate. Halogen acids convert it into monohalogen fatty acids,
and the halogens themselves convert it into dihalogen fatty acids. It
unites with aldehydes to form esters of ketonic acids, and with
aniline yields anilido-acetic acid. It forms an addition product with
acrylic ester, which on heating loses nitrogen and leaves trimethylene
dicarboxylic ester. Concentrated ammonia converts it into
_diazoacetamide_, CHN2.CONH2, which crystallizes in golden yellow
plates which melt at 114 deg. C. For other reactions see HYDRAZINE.
The constitution of the diazo fatty esters is inferred from the fact
that the two nitrogen atoms, when split off, are replaced by two
monovalent elements or groups, thus leading to the formula

N \
.. > CH.CO2C2H5, for diazoacetic ester.
N /

_Diazosuccinic ester_, N2.C(CO2C2H5)2, is similarly prepared by the
action of nitrous acid on the hydrochloride of aspartic ester. It is
decomposed by boiling water and yields fumaric ester.