1 8 INTR OD UCTION. 



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 make them alter 

 their relative positions. We see, consequently, that some form of 

 attraction or attractive power is acting between 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 solid bodies 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. 



Force may be defined as the action of one body upon another 

 body, or as the action of particles of matter upon other particles 

 either of the same or of another body. Strictly speaking, we may 

 say that facca .is.the-_cause tending to produce, change, or arrest 

 motion ; or it is any action upon matter changing or tending to change 

 its form or position. 



In many cases force manifests itself as an attractive power; for instance, in 

 the case of cohesion mentioned above, but also in adhesion, gravitation, etc. 

 Forces often give rise to motion (as in the case of heat and electricity), and 

 also to a great variety of changes in matter. The three different states of 

 aggregation are due to the relative intensity of two opposing forces, one that 

 of molecular attraction which tends to draw the molecules together, and a 

 second one that of heat which tends to separate the molecules from one 

 Another. 



Energy of a body is its capacity of doing work,^and is measured by the pro- 

 duct of the force acting and the distance through which it acts. 



Crystals are solid substances bounded by plane surfaces symmetri- 

 cVlly arranged according to fixed laws^N In explaining the formation 

 of crystals we have to assume that the particles are endowed with the 

 power of attracting one another in certain directions, thereby building 

 themselves up into geometrical forms. 



The first^cQndition essential to the formation of crystals is the possi- 

 bility of freejnotion of the smallest particles of the matter to be crys- 

 tallized; 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 



1 It will be shown later that all matter is supposed to consist of smallest particles, which we 

 call molecules. 



