Page:Popular Science Monthly Volume 29.djvu/58

48 Brazilian diamond is preferred. A crystal of hematite (iron-ore) is shown in Fig. 2; it occurs in the Island of Elba, has an iron-black color and metallic luster, while its powder is reddish-brown like ordinary iron-rust. Fig. 3 is a crystal of calcite remarkable for its optical property of double refraction and its ready cleavability in certain directions; in substance it is the same as ordinary marble; in fact, the latter consists of microscopic crystals of calcite. In Fig. 4 we have a crystal of garnet, not unfrequently seen in the mica-slates of New York. A crystal of sulphur from Girgenti, Sicily, is shown in Fig. 5; that locality abounds in fine transparent crystals of this substance. Fig. 6 represents a cube



of native silver as found in Konigsberg, Norway; and, finally (Fig. 7), a crystal of cassiterite (tin-ore) from Cornwall, in England, which has also been discovered in this country in the Black Hills, Dakota Territory. There are seven systems of crystallization, differing in the relative magnitudes and directions of certain lines of symmetry, termed the axes of the crystal. In the first, second, and third systems, these lines bear the same inclination to one another, but their magnitudes are respectively equal in the first system (see Fig. 8) (here A A', B B',



C C,' are the three axes equal in magnitude and inclined at right angles to one another), equal in two of them in the second or dimetric system (Fig. 9) (here A A' equals BB', but C C' is different from these), and unequal in all three axes in the third or trimetric system (Fig. 10) (here the axes A A', B B', and C C, are all of unequal magnitudes, but their mutual inclinations in this as well as in the second system are equal).