xxx.1 CLASSIFICATION. 687 



regard to the other axes ; thus the forms produced are 

 symmetrical or regular, and the class is called the Regular 

 System of crystals. It includes a great variety of substances, 

 some of them being elements, such as carbon in the form 

 of diamond, others more or less complex compounds, such 

 as rock-salt, potassium iodide and bromide, the several 

 kinds of alum, fluor-spar, iron bisulphide, garnet, spinelle, 

 &c. No correlation then is apparent between the form of 

 crystallisation and the chemical composition. But what 

 we have to notice is that the physical properties of the 

 crystallised substances with regard to light, heat, electricity, 

 &c., are closely similar. Light and heat undulatious.wher- 

 ever they enter a crystal of the regular system, spread with 

 equal rapidity in all directions, just as they would in a uni- 

 form fluid. Crystals of the regular system accordingly 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 examine 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 corre- 

 lated with that of crystallisation in the regular system, and 

 as soon as we know that the primary form of a substance 

 is the cube, we are able to infer with approximate certainty 

 that it possesses all these properties. The class of regular 

 crystals is then an eminently natural class, one disclosing 

 many general laws connecting together the physical and 

 mechanical properties of the substances 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 the axes are equal, but 

 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 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 principal axis, but more or 

 less rapidly in the direction of this axis, so that the iso- 

 thermal surface is an ellipsoid of revolution round that axis 



