540 S. Taker — Growth of Crystals. 



downward extension of the new growth, the smooth flat face 

 of the original crystal in the center of the hollow remaining 

 unchanged. By coloring the original crystal with chrome alum 

 the new growth could easily be distinguished from the old 

 when the crystal was sectioned for examination. 



The terraces on the under side of crystals are probably 

 formed by the repetition of crystal faces, frequently they 

 appear as minute striations parallel to the edges of the base, 

 and sometimes thay are unrecognizable, giving place to almost 

 smooth sloping surfaces. 



The experiments with weighted and unweighted crystals 

 prove : (1) that a growing crystal resting on a smooth surface 

 can raise itself and even a considerable additional load by 

 building downward the advancing outward edge of the crystal; 

 (2) that no material is deposited on the under side of the 

 crystal except along the advancing outer edge ; and (3) that 

 the cavities are not formed by solution, but are due to the 

 downward growth of the outer edge, while at the same time 

 no material is being deposited on the under surface within this 

 outer edge. 



While it is true that there may be slow diffusion of mole- 

 cules through solids and even through crystals, there is no 

 ground for the belief that a crystal like an organism can grow 

 from within. Ail of the experiments conducted by the writer 

 support the theory that a crystal grows only through the 

 addition of layers of material from without. If this is true, it 

 follows that a crystal can grow only where it is in contact with 

 a solution which furnishes it with the material necessary for 

 growth. When crystals grow in contact with glass surfaces, 

 as in the experiments described above, there is always a thin 

 layer or film of solution between the crystal and the glass. 

 The presence of this thin layer of solution is due to capillary 

 attraction and adsorption. When the surrounding solution is 

 more concentrated than the layer between the crystal and 

 glass, diffusion is set up, tending to equalize the concentration, 

 and, if the solution is supersaturated with respect to the base 

 of the crystal, the crystal molecules present in excess of satura- 

 tion are oriented and attached to the crystal as soon as they 

 come wnthin range of the molecular forces controlling crystal 

 growth — a circumstance that must occur in such limited space 

 not far from the outer edge of the crystal. 



The area of contact between a crystal and the supporting 

 surface is under greater pressure than the rest of the crystal, 

 since it must bear the weight of the crystal as well as any 

 added load. The solubility of most substances in aqueous 

 solutions increases with the pressure, and therefore the sup- 

 porting surface of a crystal of such a substance has a greater 



