Crystallization. 359 



least during growth. It is not so easy as it might appear to find a di. 

 viding line between crystals and vegetable and animal organisms. Both 

 of them as a general thing reach a limit of growth, which arises as a 

 direct consequence of growth. There are apparent exceptions to this, 

 but not real ones. A tree may continue to grow for a thousand years 

 and it does not stop as long as it lives. Some reptiles grow as long as 

 they live. There does not seem to be any reason why such a crystal as 



a hail-stone might not continue 

 growing as long as material is sup- 

 plied, since the crystals when reg- 

 ular, as in fig. 157, are placed side 

 by side. But there is always a 

 limit. If a tree planted at the 

 beginning of the Eocene Period had 

 possessed a monopoly of all the 

 carbon dioxide on earth, it would 

 by this time have had all the car- 

 bon consolidated in its tissues, 

 and it would have to stop growing 

 for want of material, just as a 

 crystal stops when all the material 

 in the bath is exhausted. But 

 aside from this there are other limiting causes. Growth is caused by the 

 deposit of materials in a definite way by polar energies. When a cer- 

 tain amount is deposited, the crystal gets into shape which changes its 

 polarity, or some other condition becomes a bar to further growth. A 

 hail-stone might get to be as big as an iceberg if it could remain in the 

 air long enough. An animal or vegetable organism is wasted as well as 

 built up by the energies which operate it, and when the amount wasted 

 is equal to that used in building up there is a cessation of growth. 

 Even in the matter of reproduction the principal is the same in crystal 

 and animal. Jf } r ou wish to start the growth of a crystal, drop into an 

 appropriate solution a small fragment of crystal as a seed, and the adult 

 crystal grows from that as a nucleus. As we have seen in chap. 33, it 

 is a fragment of the parent that becomes the nucleus of the offspring. 

 The crystal appears to have the advantage in reproduction for it may 

 start in a solution in which no "'germ " of a crystal can be detected. 

 But the germ is there all the same, for every molecule, infinitesimal as it 

 is, has its definite form and polarity, and by the polar currents set up in 

 the solution is impelled toward neighboring molecules, and their ag- 

 gregation constitutes the finished body. An animal must be regarded as an 

 aggregation of a number of different sorts of crystals, or the colloid 

 products of crystals. Each of these different sorts must have its nucleus 



FIG. 157. -Section through a Hail-stone 

 showing its crystalline structure. 



