18 INTRODUCTION. 



Solid state. Solids are distinguished by a self-subsistent 

 figure. A solid substance forms for itself, as it were, a casing 

 in which its smallest particles 1 are enclosed. The question 

 arises, by what means are these particles connected, how are 

 they kept together ? No other answer can be given, than that 

 the particles themselves attract each other to such an extent 

 that force is necessary to separate them. We see, consequently, 

 that some form of attraction or attractive power is acting be- 

 tween the particles of a solid mass, and we call this kind of 

 attraction cohesion, to distinguish it from other forms of attraction. 



The external appearance or the figure of a solid body is various. 

 It may be an irregular or a natural regular figure. Of these two 

 forms, only the latter is here of interest, as it includes all the 

 different crystallized substances. 



Crystals are solid substances, bounded by plane surfaces sym- . 

 metrically arranged according to fixed laws. 



The first condition essential to the formation of crystals is the 

 possibility of free motion of the smallest particles of the matter 

 to be crystallized ; in that case only will they be able to attract 

 each other in such a way as to assume a regular shape or form 

 crystals. Particles of a solid mass can move freely only after 

 they have been transferred to the liquid or gaseous state. There 

 are two different methods of liquefaction, viz., application 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). 



Not all matter can form crystals ; some substances never have 

 been obtained in a crystallized state, and these are known as 

 amorphous bodies (starch, gum, glue). 



Some substances capable of crystallization may also be ob- 

 tained in an amorphous state (carbon, sulphur). Other sub- 

 stances 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 



1 It will be shown later that all matter consists of smallest particles. 



