4 8 



THE POPULAR SCIENCE MONTHLY. 



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 micro- 

 scopic 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 sul- 

 phur from Girgenti, Sicily, is shown in Fig. 5 ; that locality abounds 

 in fine transparent crystals of this substance. Fig. 6 represents a cube 



Fig. 4. 



Fig. 5. 



Fig. 6. 



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 Ter- 

 ritory. There are seven systems of crystallization, differing in the rela- 

 tive 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', BB', 



C 



A 



Fig. 7. 



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 CC 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). 



