CHAP. X.] ORGANIZATION. Ill 



spheres. It may be worth mentioning that hailstones have 



been found with a structure like this, or, to use a more Hailstones. 



familiar though less really apposite comparison, composed 



of coats like those of an onion. This structure is probably 



due to their being attracted and repelled back and forwards 



between two oppositely electrified clouds, like the pith 



balls used in electrical experiments. 



The second kind of formative principle is that to which Second 

 the form and structure of crystals are due. This is different ducin'?^° 

 from the former, and much more complex. It is evident crystals, 

 that the force which brings the particles of a salt or other 

 crystallizing substance together from the water in which it 

 is in solution, must be an attractive force. But it is not 

 simply attractive. A simply attractive force, when acting 

 alone, can, as already remarked, produce none but spherical 

 forms. It is, in the largest sense of the word, a polar 

 force ; that is to say, a force that acts differently in different 

 directions : for crystals are never spherical, but are, when 

 normally formed, always bounded by plane surfaces. 

 There is also this difference, that in spherical forms the having 

 structure, when there is any, depends on the form ; but in ^"g^j^ 

 crystalline forms the form depends on the structure. This, structure, 

 as to crystals, may need explanation. In crystallization 

 the essential point, as stated in the chapter on that subject, 

 is not the form, which is variable for the species, but 

 the relation between the axes, which is constant, and 

 which constitutes the structure ; and any form is possible 

 in a species if the angles at which the planes are 

 inclined on each other are such as to be consistent with 

 the relations between the axes of that species. The 

 dependence of form on structure may be thus experi- 

 mentally illustrated. A mineralogist finds a lump of some 

 mineral substance that presents no definite form ; but from 

 its lustre, or some other characteristic, he suspects that it 

 is crystalline ; and by a few properly-directed blows of 

 his hammer he breaks it in the direction of its cleavage, 

 splitting it into large fragments bounded by faces which 

 are parallel to the cleavage planes, and make angles on 

 each other that are consistent with the relation between 



