such an extent as to disguise the species. It is then necessary to make such tests as experience has proved are requisite in order to identify the mineral. Though the aid of an elaborate series of determinative tables is usually sought, it is always wise to make a few preliminary tests in order to learn what are some of the most prominent of the physical properties of the mineral. Quite often valuable suggestions as to the character of the mineral, and at times its identity, may be learned by testing its hardness, streak, or cleavage, or by taking its specific gravity, or by observing its crystal form, structure, lustre, color, and feel.

Hardness is a factor of prime importance in the determination of minerals. The number of exceedingly hard minerals is very small. If a mineral is not scratched by topaz, it must be some one of a dozen minerals, and these are usually differentiated with ease by other simple physical tests. Ruby is thus separated from garnet, while, if its crystal form is distinct, it may readily be distinguished from the very similar ruby spinel, which is nearly as hard. A knife-blade is all that is needed to prove that iron pyrites, or "fool's gold," is not true gold, for the latter is readily scratched by the blade, while pyrites is not. It is an excellent rule to invariably test the hardness of a mineral before making any other tests.

Streak (q.v.) is of importance with colored minerals and those having metallic lustre. The scarlet streak of cochineal-red cinnabar will instantly distinguish it from cuprite, which has the same color but a brownish-red to blood-red streak. The brown streak of manganite serves to distinguish it from pyrolusite, which has a black streak, while a test of their hardness shows that manganite is 4 and pyrolusite 2 to 2.5. The distinction between gold and pyrite is very marked, gold having a streak of the same color as the mass, while the streak of pyrite is greenish-black. This same greenish-black streak is, however, characteristic of copper pyrites, and its external color is quite similar to that of iron pyrites, but they are readily separated by the superior hardness of the iron pyrites. Such tests as these are not only most convenient in field work, but are at times conclusive. (See MINERALOGY; PHYSICAL CRYSTALLOGRAPHY.)

Cleavage is often an important aid in determinative work. If a lead-gray mineral of metallic lustre shows well-developed cubical cleavage, it may be safely assumed that it is galena. If a transparent mineral cleaves readily into octahedrons of large size, it is almost certainly fluorite. Sphalerite is the only mineral in which dodecahedral cleavage is highly perfect, while the excellent rhombohedral cleavage of calcite instantly distinguishes it from quartz, which it often resembles, and the separation is made conclusive by testing the hardness, which in calcite is 4 and in quartz 7.

Crystal form is one of the most important determinative factors. The characteristic cross twins of staurolite suffice to determine that species, which it would be difficult to recognize if massive. The trillings of cerussite easily separate it from anglesite, whose physical properties are very similar. Many of the zeolites and their common associates are recognized by their crystal form; thus analcite is isometric and generally in trapezohedrons, chabazite is

rhombohedral, apophyllite is tetragonal. The "gridiron" structure of microcline as seen in thin sections under the microscope, which is due to twinning, serves effectually to differentiate it from all of the other feldspars. The importance of a knowledge of crystallography (q.v.) as an aid in determinative mineralogy cannot be overestimated.

Specific gravity is one of the least variable properties of minerals. Its accurate determination, therefore, may be the only test needed to positively identify a specimen. This fact is of immense importance to the jeweler, who constantly resorts to his specific gravity balance as his safest test for precious stones. Gold, too, may be thus identified, for no other yellow mineral has a specific gravity approaching it. Even the rough test of placing the unknown mineral in the palm of the hand and comparing it with about the same quantity of a known species may be suggestive as to the identity of the mineral. Thus rusty-yellow quartz sand may be instantly distinguished from the very similar but much heavier monazite sand.

While the examination of the physical prop erties of an unknown mineral, as above suggested, is of great value, there is but one way in which its identity can be fixed in all cases, and that is by the use of a series of determinative tables in which the mineral is referred successively from one large group to a smaller, until finally all have been eliminated but itself. The process is based largely on a series of chemical tests which, in almost all cases, give an insight into the character of the materiai. The scheme involves (1) the heating of a fragment in the forceps; (2) heating in a closed tube; (3) heating in an open tube; (4) heating on charcoal in the blowpipe flame; (5) fusion with fluxes on platinum wire. These processes are described under BLOWPIPE ANALYSIS (q.v.).

While described species may be determined by the foregoing methods, it is at times desirable, and in the case of new species essential, to ascertain the exact proportions of the various ingredients which are present in the mineral; for this purpose it is necessary to resort to a complete quantitive analysis. See CHEMICAL ANALYSIS.

Bibliography- Penfield's revision of Brush's 'Manual of Determinative Mineralogy'; Endlich, Manual of Qualitative Blowpipe Analysis and Determinative Mineralogy); Frazer, Tables for the Determination of Minerals by Physical Properties; Crosby. Tables for the Determination of Common Minerals.'

GEORGE LETCHWORTH ENGLISH, Mineralogist, New York City. Determinate Problem, a problem in geometry which admits of a limited number of solutions, an indeterminate problem admitting of an indefinite number of solutions.

Deter'minism, a name applied by Sir William Hamilton to that system of philosophy which holds that the will is not a free agent, but is irresistibly determined by providential motives, that is, by motives furnished by Providence, which turn the balance in our mental deliberations in accordance with its views. Determinism is often mistakenly confused with materialism and atheism. It does not deny moral responsibility though holding a man cannot choose to act in a certain way while he

prefers with his whole nature to act in another way. This doctrine of the necessitarian philosophers expounds in short that whatever happens is entirely determined by antecedent causes. Detlef, Karl, kärl dět'lěf, pseudonym of Klara Bauer, German novelist: b. Swinemünde 23 June 1836; d. Breslau 29 June 1876. Her Indissoluble Bonds' (1877) and Must It Be? (1872), are valuable and engrossing studies of

character.

Detmold, dět'mōld, Germany, city, capital of Lippe, on the Werra, 50 miles southwest of Hanover. It consists of an old and a new town, the former poorly, the latter regularly built. Its principal edifice is the palace, a fine old castellated building, with a vast round donjon tower. In the vicinity, on the Grotenberg, the loftiest summit of the Teutoburger Wald, a colossal statue, 45 feet high, placed on a solid circular pedestal twice that height, has been erected to the Hermann or Arminius who overthrew Varus and his legions. Pop. 11,542.

Det'onating Powder, certain chemical compounds, which, on being exposed to heat or suddenly struck, explode with a loud report, owing to one or more of the constituent parts suddenly assuming the gaseous state. A mixture of equal volumes of chlorine and hydrogen exposed to direct sunlight detonates violently, forming hydrochloric acid gas. The chloride and iodide of nitrogen are very powerful detonating substances. The compounds of ammonia with silver and gold, fulminate of silver and of mercury, detonate by slight friction, by means of heat, electricity, or sulphuric acid. The compound used in the priming of percussion-caps and fuses is the fulminate of mercury or silver, collected as a precipitate when the metal, dissolved in nitric acid, is poured in warm alcohol and then collected, washed, and dried.

Det'onators are the devices used in firing high explosives by detonation. They consist of small cylindrical copper tubes closed at one end and charged with mercuric fulminate or a mixture of mercurial fulminate and potassium chlorate which is compressed in the bottom of the tubes. Those used for blasting in mines and quarries are also known as blasting caps and exploders. They are rated as single force, double force, triple force and so on, the charge for the single force cap being about five grains of the detonating substance and the charges for the higher force increasing about two grains for each grade. To fire them a piece of Brickford or "running" fuse of the desired length is inserted in the mouth of the detonator or cap and then the copper tube is bent or "crimped» securely about the sides of the fuse. The detonator is inserted in the bore hole so as to come in close contact with the first cartridge or "stick" of explosive and the bore hole is filled with tamping. When a flame is applied to the end of the fuse that projects from the bore hole, the column of powder in the fuse takes fire, the fire travels slowly down to the charge in the detonator, causing the detonation of the dynamite.

For military mines and naval torpedoes and, to a considerable extent, for commercial blast ing, detonators to be fired by an electric current are employed. These are known as electric detonators. They differ from those previously described only in that the mouth of the copper

cap is closed by a plug made of sulphur and ground glass, through which two copper wires, known as the "legs of the detonator," are led. Inside the cap these wires are bridged over by a very fine wire made of an alloy of iridium and platinum. Around the bridge and between it and the fulminating composition is placed a layer of mealed gun-cotton. When it is desired "to fire," the legs of the detonator are connected with a dynamo-electric machine and the current generated. As the current passes it heats the bridge to incandescence, which sets fire to the guncotton; this causes the fulminate to detonate, and this detonates the charge of dynamite or other high explosives. The electric detonators used in the United States navy contain 35 grains of mercuric fulminate, this large quantity being used to make sure that the detonator will do its share of the work in torpedo attacks on an enemy. See DETONATING POWDER; Ex

PLOSIVES.

Detri'tus, applied in geology to accumulations formed by the disintegration of rocks, may consist of angular and sub-angular debris, or of more or less water-worn materials, such as gravel, sand, or clay, or an admixture of these. Especially applicable to that fragmentary matter which if consolidated into a solid would form what is known as breccia.

De Trobriand, de trō-bre-än, Philippe Regis, BARON, American soldier: b. Tours, France, 4 June 1816; d. Bayport, Long Island, 15 July 1897. He was educated at the University of Orleans, studied law at Poitiers, and came to the United States in 1841. He was commissioned colonel of the 54th Volunteers in the Civil War 1861; and was appointed brevet brigadier-general in 1867. He was editor and publisher of the 'Revue du Nouveau-monde,' New York, 1849-50, and joint editor of the Courier des Etats Unis 1854-61. Among his 'Les Gentilshommes de l'Ouest' works are: (1841); Quatre ans de campagnes a l'Armee du Potomac (1867).

Detroit (Fr. détroit, strait), Mich., capital of Wayne County, in the southeast; largest city of the State, and of the entire Northwest to the Pacific; 13th of the United States; on the northwest bank of the Detroit River dividing the United States from Canada, one end six miles below Lake St. Clair and the other about 14 from Lake Erie. It is 88 miles from Lansing, the State capital; 284 from Chicago; 60 from Toledo, Ohio; 251 from Buffalo; and 291 from Mackinaw. Pop. (1900) 285,704; (1903) about 325,000; (1910) 465,766.

Detroit has the finest harbor on the Lakes; the river, on the city front, is often called "the Dardanelles of America." The broad outlet of Lake St. Clair, running west and dividing around Belle Isle, narrows to about half a mile and deepens to an average of 32 feet for some miles with a southwest course, before turning directly south, with a current of about two miles an hour. Fed by the Great Lakes, it has always a full stream, neither rising nor sinking much, and is little disturbed by storms; and the largest vessels can lie up to the wharves. Here is built Detroit, extending some seven miles along the river front, lined with wharves, elevators, foundries, warehouses, railroad stations, and freight depots, etc.; and about three miles deep, except in the centre, where it is nearly