The germ presumably is an assemblage of parallel molecules, therefore possesses a larger attractive force than the surrounding particles, and if this force exceeds the internal friction will draw to itself and orientate other particles.
Frequently the growth, as observed on a plate of glass on the microscope stage, is rela tively rapid in one direction, suggesting a pointed rod pushing its way rapidly into the richer solution, but thickening slowly, because of the impoverished zone surrounding it. As this zone isgradually enriched by the currents, secondary offshoots form and grow in import ant crystal directions and these in turn send off tertiary offshoots and so on, the tendency being to the creation of skeleton outlines of crystals. As the concentration of the solution decreases the process becomes slower, the hollow places fill and the complete crystals result.
It is sometimes claimed, as showing an analogy between crystal growth and the growth of an organism, that every species of crystal has a definite limit of growth, and that when this is passed new crystal individuals are formed and the old individual grows no further. Exact measurements show that the large crystals do continue to grow, although with the larger sur face the growth is less evident. Moreover, if the cooling or evaporation is. made sufficiently slow no new crystals are formed, but all the material, deposits on the-already formed crystals.
Why a substance should vary in the occur ring faces of its crystals is not understood. Rapid formation often results in simple forms and slow formation in complex forms. Foreign material in small amounts, if it crystallize with the substance, may cause the development of unusual faces and the simple presence of large amounts of foreign material may have a similar effect.
In conclusion, it may be said the study of crystals is no longer a simple study of external form, but involves of the highest in terest to the physicist, chemist and geologist. The purely geometric portion, both as relating to varieties of regular structure and to the laws governing the association of faces in the ex ternal form, is well advanced, possibly nearly completed. But many facts remain to be ex plained as to origin, growth, habit, nature of molecules and molecular forces and the rela tions between chemical constitution and struc ture, See CRYSTALLINE; CRYSTALLOGRAPHY; CHEMICAL CRYSTALLOGRAPHY; GEOMETRICAL CRYSTALLOGRAPHY; PHYSICAL CRYSTALLOG RAPHY; MINERALOGY.
Bibliography.— Baumhauer, H., (Die nenere Entwickelung der Krystallographie' • Bragg, W. H. and W. L., (X-Rays and Crystal Stru6 ture' (London 1915) ; Goldschmidt, V., das Wesen der Kristalle Ostwalds, Annalen der Naturphilosphie (Bd. 9); Lehmann, O., (Fliissige Kristalle' (Leipzig 1904)Miers, H. A. and Isaac F., (On Crystal Growth' (Proceedings Royal Society, London, 79, 322, 1907, and Journal Chemical So ciety, London, 89', 413, 1906) ; Schiinflies, A., and Krystallstructur) (Leip zig 1891) ; Siihnke, L., (Entwickelung einer Theorie der Krystallstnictur) (Leipzig 1879) ; Story-Maskelyne and Others, (Report Commit tee of British Association for Advancement Science' (1901) ; Tutton, A. E. H., (Crystals) (London 1917).