5ZS2 Professor Necker on Mmerahgy considered 



Hect. tl. — Minerals considered according to their physi- 

 cal PROPERTIES. 



A. Of the form of mineral individuals, or of crystallization. 



§ 21. Definition of a crystal ; its parts; fundamental or pri- 

 mitive, and derived or secondary forms. Systems of funda- 

 mental forms, with the derived forms belonging to each system* 

 Laws of derivation. Mode of determining the relative dimen* 

 sions of fundamental forms. Mode of determining the position 

 of derived planes, in regard to the planes or to the axis of the 

 fundamental form, and the incidences of the derived planes on 

 the fundamental planes, or on each other. Cleavage. Optical 

 properties of the different systems of forms, and means of ascer- 

 taining these properties. Relation existing betwixt the form and 

 the chemical composition of minerals. Theory of isomorphism. 

 Mode of grouping of simple crystals or individuals. Compound 

 crystals or individuals. Macles or hemitrope crystals. Regu- 

 lar groups of crystals. Irregular groups of crystalline particles 

 or distinct concretions, &c. 



B. Of the optical phenomena unconnected mth the form. 



§ 22. Transparency, lustre, colour, phosphorescence, &c. 



C. Of other physical properties, 



§ 23. Specific gravity. Hardness. Nature of the streak 

 and of the dust. Electricity. Magnetism. Taste. Feel. 

 Sound, &c. 



IIL—TAXONOMY, or THEORY OF CLASSIFICATION. 



Sect. I. — Of the choice op proper characters among the 



VARIOUS properties OF MINERALS. 



§ 24. We cannot employ as characters, either distinctive or 

 descriptive, of mineral individuals, all the facts, phenomena, and 

 properties enumerated in the preceding chapter. It is owing to 

 the promiscuous and indiscriminate use of them that mineralogy 

 has been considered more as an abstract science than as a de- 



