FUNDAMENTAL PROPERTIES OF MATTER. 25 



The crystals are the most unregular of all, having three unequal axes, all 

 intersecting at oblique angles. The fundamental forms of this system are the 

 triclinic prism and the triclinic octahedron or double pyramid (Figs. 11 and 

 12). Some substances crystallizing in this system are copper sulphate, potas- 

 sium dichromate, gypsum, boric acid, manganous sulphate. 



The pyramidal form is found in all the systems, while the cube is confined 

 to the regular system, and prisms occur in all but the regular system. 



FIG. 11. FIG. 12. 



Triclinic prism. Triclinic octahedron. 



For some reason or other crystals sometimes grow mainly in one or two 

 directions and then show forms that are distorted and have little or no resem- 

 blance to the normal forms of the system to which they belong. Examples of 

 such forms are the following : 



Tabular crystals or flat plates, as potassium chlorate, iodine, etc. 



Laminar crystals, thin plates or leaflets, as acetanilid, naphthol, calcium 

 hypophosphite, etc. 



Acicular crystals or needles, as aloin, cinchonidine sulphate, quinine 

 salts, etc. 



Prismatic crystals or prisms, extended chiefly in the direction of the longest 

 axis, as salicylic acid, santonin, cinchonine sulphate, etc. 



The shapes of gems must not be confused with crystal forms. They are the 

 result of cutting and polishing for the purpose of causing the gem to reflect 

 more light. 



Characteristic properties of solids. Solid substances show a 

 great variety of properties caused by the differences in the cohesion 

 of the particles (molecules) composing the substances, and accordingly 

 we distinguish between hard and soft, brittle, tenacious, malleable, 

 and ductile substances. 



Hardness is that property in virtue of which some bodies resist attempts to 

 force passage between their particles, or which enables solids to resist the dis- 

 placement of their particles. Diamond and quartz are extremely hard, while 

 wax and lead are comparatively soft. 



