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system, weiss and cleavage

MINERALOGY (-al'-), in natural his tory, a science treating of those natural inorganic products of the earth which possess definite physical and chemical characters. In 1669 Nicolas Steno, a Dane, made the discovery that in crys tals of quartz the angles of inclination of adjoining faces were constant, and that the number of faces and their grouping, notwithstanding variations in size, were always the same. In this year also the doubly-refracting property of Iceland spar was observed. In 1772 Rome de l'Isle announced that the various shapes of crystals of the same product were in timately related. The Abbe HMV in 1784 discovered that 10 forms, including six of de could be produced from various minerals by cleavage, and that these must be the true primitive forms. Professor Weiss, of Berlin (1809-1815), established fundamental lines, which he called axes, and to which he showed how all the primitive forms and second ary planes were related. Subsequently, though independently, Mohs (1820-1825) arrived at a division of crystals into four systems of crystallization which coin cided with the four axial groups of Weiss. He also announced two other systems of crystallization, in conse quence of more precise measurements being obtainable by the use of the re flective goniometer. The discovery by

Malus in 1808 that a ray of ordinary light reflected at a certain angle from a glass plate possessed the same prop erties as that which emerged from Ice land spar, enabled Brewster in 1819 to point out the intimate relation which ex isted between the cleavage form of a mineral and its action on light.

The early attempts at classification were very vague, and were founded on supposed external differences, being di vided into Earths, Stones, and Metals. Cronstedt's "Essay" (1758) was the first foreshadowing of a principle in a system of classification. The earths he classed as Calcareous, Siliceous, Argillaceous, and so on. Werner's last system divided fossils (as minerals were then called) into four classes: viz., Earthy, Saline, Combustible, and Metallic. The system of Haiiy (1801), like that of Werner, was a mixed one, but it was the first to di rect attention to the importance of crys tallographic form to a system of classi fication.