20 CHEMICAL PHYSICS. 



Energy may be potential (i. e., stored up) or kinetic (i. e., actual). For 

 instance, potential energy is the energy which we have in a mass held by 

 the hand, or by a support ; as soon as the support is withdrawn the mass falls, 

 and in this instance we witness kinetic or actual energy. 



Other instances of potential energy are a drawn bow, a wound-up watch- 

 spring, an elevated tank of water, etc. This potential energy may manifest 

 itself as kinetic energy by sending an arrow through space, by keeping the 

 watch in motion, or by rotating a water-wheel. During the conversion of 

 potential into kinetic energy there is neither gain nor loss ; both are absolutely 

 alike in quantity. 



Crystallization. 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. 



Crystals are solid substances bounded by plane surfaces symmetri- 

 cally arranged according to fixed laws. 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 external form of a crystal is only an outward expression of a regular 

 internal structure. This is shown by the fact that in non-crystalline homo- 

 geneous bodies such properties as elasticity, hardness, cohesion, transmission 

 of light, etc., are the same in all directions, while crystallized bodies show dif- 

 ferences along different directions. A model of glass would not be a crystal, 

 since the necessary internal structure is absent. 



The first condition essential to the formation of crystals is the possi- 

 bility of free motion 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 

 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). 



In the great majority of cases the method employed for obtaining crystals 

 is to dissolve the substance in a liquid, usually water, taking advantage of the 

 fact that, with very few exceptions, substances are more soluble in hot than in 

 cold liquids. When such a concentrated hot solution, filtered if necessary to 

 remove solid matter, is allowed to cool, the particles of the excess of the sub- 

 stance, beyond what is soluble at the lower temperature, gradually arrange 



