CLASSIFICATION. 361 



crystallization and the chemical composition. But what 

 we have to notice is that the physical properties of the 

 crystallized substances with regard to light, heat, elec- 

 tricity, &c., are closely similar. Light and heat undula- 

 tions, wherever they enter a crystal of the regular system, 

 spread with equal rapidity in all directions, just as they 

 would in a uniform liquid, gas, or amorphous solid, such 

 as unstrained glass. Crystals of the regular system accord- 

 ingly do not in any case exhibit the phenomena of double 

 refraction, unless by mechanical compression we alter the 

 conditions of elasticity. These crystals, again, expand 

 equally in all directions when heated, and if we could cut 

 a sufficiently large plate from a cubical crystal, and ex- 

 amine the sound vibrations of which it is capable, we 

 should find that they indicated an equal elasticity in 

 every direction. Thus we see that a great number of 

 important properties are correlated with that of crys- 

 tallizing in the regular system, and as soon as we know 

 that the primary form of crystallization of a substance is 

 the cube, we are able to infer with approximate cer- 

 tainty that it possesses all these properties. The class 

 of cubical crystals is then an evidently natural class, 

 one disclosing general laws connecting together the 

 physical and mechanical properties of the substances so 

 classified. 



In the second class of crystals, called the dimetric, 

 square prismatic, or pyramidal system, there are also 

 three axes at right angles to each other, two of which are 

 equal, and the third or principal axis is unequal, being 

 either greater or less than either of the other two. In 

 such crystals accordingly the elasticity and other physical 

 properties are alike in all directions perpendicular to the 

 principal axis, but vary in all other directions. If a point 

 within a crystal of this system be heated, the heat spreads 

 with equal rapidity in planes perpendicular to the prin- 



