A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . directions; the action of light,the electric reactions, the rate of decay,is dissimilar in different directions, and sim-ilar in others, and the inference in unavoidablethat crystals are molecularly arranged withregard to fixed directions. These directionsare called axes. In undertaking to reach primitive or par-ent forms of crystals it is clear at the outsetthat they include prisms and pyramids, thatthese can have bases that are square
A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . directions; the action of light,the electric reactions, the rate of decay,is dissimilar in different directions, and sim-ilar in others, and the inference in unavoidablethat crystals are molecularly arranged withregard to fixed directions. These directionsare called axes. In undertaking to reach primitive or par-ent forms of crystals it is clear at the outsetthat they include prisms and pyramids, thatthese can have bases that are squares, or rhomboids, or even polygon-al (many sided) that the prisms and pyramids can be erect or in-clined. Furthermore in the investigation of crystallized minerals,by developing forms through cleavage, minerals are found whichgive dodecahcdral forms (PI. i. Fig. 36). By this method of in-quiry some thirteen parent forms at first appeared to underlie thepossible number of derivative crystals; eight prisms with four-sidedbases, five erect, three inclined; a six-sided erect prism; three octa-hedrons ; and a dodecahedron. But the method of cleavage revealed. A POPULAR GUIDE TO MINERALS the fact also that forms like the cube, the equal-sided octahedron,and the dodecahedron, are related; that the same minerals yieldthem, and that upon an assumption of three equal and rectangularaxes they could be brought together under one crystallographic defi-nition, and with them a new number of derivative forms. Furtherreflection and observation made it apparent that all crystals couldbe referred to six systems, which were separated by the relations ofcertain directions, called axes, which axes formed the structuralbasis of these systems. These axes were lines of force, of accretion,and were symptomatic of a deeper molecular orientation in the ul-timate particles of the crystal. Fig. 6 shows an incomplete crys-tal of silver in which along the direction of three lines, at rightangles to each other,
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
