EXTENSION OE FIGURE. 19 



to the liquid or gaseous state. There are two different methods of 

 liquefaction, viz., by means of heat (melting), or solution in some 

 suitable agent (dissolving). In the liquid condition thus produced, 

 the smallest particles can follow their own attraction, and unite to 

 form crystals on removal of the cause of liquefaction (heat or solvent). 



If two or more (non-isomorphous) substances for instance, common salt 

 and Glauber's salt be dissolved together in water, and the solution be allowed 

 to crystallize, the attraction of like particles for one another will be readily 

 noticed by the formation of distinct crystals of common salt alongside of 

 crystals of Glauber's salt; neither do the particles of common salt help to 

 build up a crystal of Glauber's salt, nor the particles of the latter a crystal of 

 common salt. Advantage is taken of this property in separating (by crystal- 

 lization) solids from each other, ^ 4ien they are contained in the same solution. 



Not all matter can form crystals ; some substances never have been 

 obtained in a crystallized state, such as starch, gum, glue, etc. A 

 solid substance showing no crystalline structure whatever is called 

 amorphous. 



nSbme substances capable of crystallization may be obtained also in 

 an amorphous state (carbon, sulphur). Other substances are capable 

 of assuming different crystalline shapes under different conditions. 

 Thus sulphur, when liquefied by heat, assumes, on cooling, a shape 

 different from the sulphur crystallized from a solution. One and the 

 same substance under the same conditions always assumes the same 

 shape. Substances capable of assuming in solidifying two or more 

 different shapes or conditions, are said to be dimorphous and poly- 

 morphous, respectively. When substances of different kinds crystallize 

 in exactly the same form we call them ixomorphous (sulphate of 

 magnesium and sulphate of zinc). If two isomorphous substances 

 be contained in one solution, they will crystallize together, and the 

 crystals are made up of particles of both substances. 



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. 



