CRYSTALLOGRAPHY), and its optical and other physical properties (see PHYSICAL CRYSTALL OGRAPHY) ; specimens of previously described species are accurately and rapidly determined by much simpler methods, varying from sight recognition to systematic testing with the aid of tables or (schemes." Sight recognition is the result of experience in handling and testing and has no fixed order of procedure. Except for this, it is similar to the later mentioned schemes by external signs, inasmuch as it re lies chiefly on lustre, color, crystals, cleavage, associates, etc., and supplements these by tests such as the color of the mark (streak) made by it upon unglazed porcelain, or its hardness as tested by a knife point. Testing at random is nearly worthless and in all cases of doubt it is better to follow the guidance of some system atic scheme in which a feW simple physical or chemical tests made in definite order suc cessively place the specimens in smaller and smaller groups until the group is so small that the differences between its members are read ily recognized and it becomes possible con clusively to eliminate all but one species. There are many schemes which may be said to be chiefly of three types. (1) Schemes in which the classifying characters are the so-called oexternal such as lustre, color, crystalline form and simple physical tests such as hard ness, color of powder (streak) or specific gravity. These schemes are invaluable in the classroom, enormously lessening the time spent in looking up characters and they foster acute observation of appearance and require only simple apparatus, such as a pocket lens, streak plate and file or knife. The best known and most followed scheme of this type was pre pared in 1866 by Prof. Albin Weisbach of Freiberg, and while for the great group of minerals with non-metallic lustre and white or gray streak it is not very conclusive, the min erals of metallic lustre or with colored powders (streaks) are often very rapidly determined. For instance by the Frazer-Brown modification. of Weisbach's tables only three species are at once Submetallic or non-metallic, Red streak, Hardness about 4 (that of fluorite). These are easily distinguished by the shade of red of the streak and by the specific gravity: Zincite, orange red, Cuprite, brownish-red, sp. gr. 6, Hauerite, brownish-red, sp.gr. 4; (2) Schemes in which the so-called cblowpipe tests such as fusibility or qualitatiVe determination of principal constituents, or solubility in are the classifying tests, -while the external signs and physical tests are subordinate. These, because they are largely independent of condition in which the mineral occurs, use only minute amounts of material, give positive proof of the presence of the component ele ments, may fairly be said to be of the general applicability. They require in addition. to the simple apparatus used in the previously. mentioned schemes, a blowpipe and its panying lamp, forceps, wire, reagents, etc.:
Undoubtedly the most used schemes of this type are translations and modifications of von. Kobell's 'Tables for Determination of Min erals by Simple Chemical Tests,) first published, in 1833 'Ito facilitate determination of minerals so that by simple testing with the blowpipe and in the dry way one is led quickly to a small group which includes the mineral being tested.# As with the Weisbach tables, there is first a, division by lustre, but thereafter the route fol lowed is different, the next division being by fusibility and the further subdivisions in some cases being the detection of particular elements as arsenic, sulphur, antimony, iron, etc., and in others some detail of fusion such as °magnetic after or of solubility such as °effer vescence.' For instance, a specimen of green granular Chrysolite (Mg, Fe).SiO, would be placed by the Brush-Penfield modification of the Von Kobell tables successively in the fol lowing divisions and sub-divisions: 1. Non-metallic lustre. One of several hundred.
2. Infusible. True of 130 of the non-metallic ipeeies: 3. Dissolves in HCl laving a gelatinous residue on evaporation. Therefore one of 16 species.
4. Gives reactions for Magnesium. Therefore one of 6 speriss.
2. Contains no water. Therefore ode of 2 species. 6. Gives reactions for iron. Therefore chrysolite.
This confirmed by color, hardness, structure, etc.
(3) Schemes in which characters determined by the polarising microscope are used as the classifying tests, these including single or double refraction, index of refraction, birefringence, pleochroism, Shape, cleavage, color, etc. Such schemes are extensively used for the determina tion of the minerals in fine-grained rocks and are coming into more general use for identifica tion not only of those minerals which can be readily made thin enough to transmit light but for the so-called opaque minerals. The testing requires specially prepared sections as thin as papers or finely crushed powder or, in some eases, polished sections, and the tests while usually easily made require for their proper understanding a considerable knowledge of crystal structure and optics. They, however, supplement admirably the blowpipe and physi pi tests.
Bibliography.— Works in English in addi tion to tables In most textbooks of mineralogy. External Signs: Eakle, A. S:, (Mineral Tables' •, Frazer-Brown, (Tables for the De termination of Minerals' (6th ed.); Kraus Hunt,