24 



CHEMICAL PHYSICS. 



potassium sulphate and nitrate, resorcin, zinc sulphate, citric acid, iodine, 

 Rochelle salt, mercuric chloride, barium chloride, tartar emetic, codeine, sali- 

 cylic acid, piperin, Epsom salt, silver nitrate, ammonium sulphate, cream of 

 tartar. 



(4) Hexagonal or rhombohedral system. 



The crystals have four axes, three of which are of equal length, while the 

 fourth is either longer or shorter than the other three. The three equal axes 

 are in the same plane and intersect at an angle of 60, while the fourth axis 

 intersects these at right angles. The fundamental form is the double six-sided 

 pyramid. The rhombohedron and regular six-sided prism are modifications 

 of this system (Figs. 6, 7, and 8). Some substances crystallizing in this system 



FIG. 7. 



FIG. 8. 



Rhombohedron. 



Six-sided prism. 



are sodium nitrate, camphor, graphite, ammonium chloride, ice, calcspar, thy- 

 mol, metallic bismuth and antimony, arsenic, silicic acid. 



The clinometric group includes the following systems : 



(5) Monociinic system, also known as the monosymmetric, clinorhombic, or 

 oblique prismatic system. 



The crystals have three unequal axes, two intersecting at oblique angles and 

 both intersecting the third at right angles. The fundamental forms of this 

 system are the monoclinic double pyramid or octahedron and the monoclinic 

 prism (Figs. 9 and 10). Some substances crystallizing in this system are 



FIG. 9. FIG. 10. 



Monoclinic double pyramid. Monoclinic prism. 



ferrous sulphate, borax, lead acetate, cupric acetate, tartaric acid, potassium 

 chlorate, and sodium acetate, sulphate, thiosulphate, phosphate, and carbonate. 

 (6) Triclinic system, also known as the asymmetric, clinorhombohedral, or 

 doubly oblique prismatic system. 



