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MINERALOGY 589 edies, usually of the miraculous kind. Pass- ing to the Christian era, we find Pliny writing on this subject in the 1st century, and Dios- corides in the same or the following; after which there is a blank until the llth cen- tury, when Avicenna divided minerals into four classes, stones, salts, sulphurous or in- flammable bodies, and earths. The "stones" were chiefly silicates, and rude as this classifi- cation is, it was not until long after chemical science had made its mark that anything very much superior was advanced, the principal im- provement made in more than six centuries being the substitution of metals for " stones." Agricola (1543-'50) wrote several works, stud- ied the external characters of minerals, and based his arrangement upon those which are apparent to the senses. The alchemical stud- ies of the succeeding centuries bore some fruit, both in the discovery of new species and in the addition of heating and fusion as modes of investigation. Passing over Lin- naeus (1735) and Wallerius (1747), who was the first to write a systematic descriptive work on this subject, we come to Cronstedt of Sweden, who in 1758 first pointed out the distinction between rocks and minerals which now enters into the fundamental definition of the latter. He based his system upon chemical proper- ties. Rome de Lisle, in 1772-'83, made the first systematic effort to apply the principles of crystallography to the science, though Nicho- las Steno had in the preceding century pointed out the fundamental fact that, with all their variations of form, the faces of crystals pre- served the same angular relations ; and later Gulielmini discovered that cleavage gave con- stant forms. Werner of Freiberg published in 1774 a work on the " External Characters of Minerals," in which he gave a much needed precision to the descriptive part of mineralogy, and retained the " natural affinity," or chemi- cal composition, as the grand basis of classi- fication, though the mode of carrying out the idea recalls Avicenna's work, seven centuries before. Werner also made four classes, earths, salts, inflammables, and metals, the first named being further divided into silicious, argilla- ceous, calcareous, and talcose ; but the silicious division was made to include nearly all the hard minerals, without regard to composition. It was under this form that mineralogy was introduced to English students by Kirwan in 1784. The science now began to receive con- stant and important additions, the three modes of determination which still remain criteria (crystalline form, chemical composition, and physical characters) being each in turn ele- vated to a position of dominant importance. In 1783 De Lisle published a second edi- tion, in which crystallography received in- creased attention; and in 1801 Haiiy's Traite de mineralogie appeared, in which crystallog- raphy was made the principal agent in the de- termination of mineral species. He rediscov- ered the importance of cleavage, and afforded a mathematical explanation of the phenome- non, referred the numerous secondary forms to a fundamental molecule of invariable shape, and reduced all crystal forms to six systems, based upon the following forms : 1, the regular octahedron; 2, the rhombohedron ; 3, octahe- dron with a square base; 4, the octahedron with a rectangular base ; 5, the prism with a symmetrical oblique base; and 6, the prism with an unsymmetrical oblique base. By ref- erence to the article CRYSTALLOGRAPHY it will be seen that, though the details of his system have been changed, the axial differences recog- nized by him remain. In his system chemical composition and physical characters were enc tirely subordinate to crystallographic habits. He made four classes: 1, free acids; 2, sub- stances which are metallic but do not present a metallic appearance, in which were included the eight genera, lime, barytes, strontites, magnesia, alumina, potash, soda, and ammonia, together with the silicates; 3, metallic sub- stances ; 4, unmetallic combustible substances. In 1804 Mohs of Vienna published a descrip- tion of a collection of minerals, in which the external characteristics alone were used to describe them. In 1820 he expanded his ideas into a "Natural History" system, the object of which was to group together all minerals which presented similar characters in regard to taste, lustre, gravity, streak, hardness, &c. No tests were used which destroyed the mine- ral, such as acids and fusion. Each group was gradually reduced by a process of comparison and exclusion to its individual members. This method was borrowed from other fields of science, and its nomenclature repeated the classes, orders, and genera of zoology and botany. The system, though it has proved to be entirely unfitted to this science, did much good by requiring greater precision in descrip- tion, bringing out many true relationships be- tween species, and discarding unimportant dis- tinctions which were flooding the science with false species. It is still used, with modifica- tions, in mineralogical keys which are con- structed for the use of young students, and persons little versed in the study. Mohs's classification included three classes: class 1 contained four orders, gas, water, acid, and salt, and included bodies which have taste, give no bituminous odor, and have a gravity below 3-8 ; class 2, bodies which have no taste, but are of specific gravity above 1-8; class 3, fluid bodies with bituminous odor, and taste- less bodies of specific gravity below 1 '8. This system was received with great favor, and not only held sway in Germany for 40 years, but extended into England and America. Two of the modes of determining minerals, crystalline form and physical characters, had now received the attention of able advocates, and were in rapid process of development by mineralogists throughout the world. The third, that which stands at the head in the present system, is chemical composition, which received from