This arranging, grouping, and piling up of molecules is called crystallization, and the solid formed in this way is called a crystal. Figures 70b and 70c show the shot arranged to reproduce two common forms of crystals (e.g., fluorite and calcite)." (Whitlock.) [Illustration: Fig. 70.--Piles of shot arranged to give some idea of the manner in which molecules are bound together in various crystal forms. (After Whitlock, New York Museum.)] A combination of certain facts regarding crystals furnish all but absolute proof of some sort of regularity of arrangement of particles within them. Among such facts are the following: (1) the wonderful regularity of arrangement of faces upon crystals is practically impossible to account for except as the outward manifestation of regularity of structure or systematic network arrangement of the interior; (2) most crystals split or cleave more or less perfectly in one or more directions presumably in accordance with certain layered structure of the constituent particles; (3) all of the many known forms of crystals can be accurately grouped in regard to their effects upon the passage of light (especially polarized light) through them, each kind or type of network structure presumably producing a different effect upon light; and (4) X-ray photographs have proved that particles, or at least groups of particles, are very systematically arranged within crystals. It will be instructive for us to make a comparison between the growth of crystals and organisms. Both really grow, but each species of organism is rather definitely limited in size while there is no known limit to the size which may be attained by a crystal so long as material is supplied to it under proper conditions. As a matter of fact crystals vary in size from microscopic to several feet in length, those less than an inch in length being most abundant by far. Organisms mostly grow from within, while crystals grow from material externally added. It is an astonishing fact that in crystals as well as organisms growth takes most rapidly on a wound or broken place. Thus if a crystal is removed from the solution in which it is growing and put back after a corner has been broken off, the fractured surface will build up more rapidly than the rest. Finally, crystals are not necessarily limited in age like organisms. Under certain natural conditions, as, for example, weathering, crystals may decay or be broken up; but where they are protected as constituent parts of rock formations well below the earth's surface they may remain unchanged for indefinite millions of years. Thus in a ledge of the most ancient known or Archeozoic rock only recently laid bare by erosion one may see crystals which are precisely as they were when they crystallized many millions of years ago.