GROWTH OF LARGE CRYSTALS. 75 



the expense of the sugar below, which is being- constantly taken into solu- 

 tion and deposited as crystals about the string; and, therefore, although the 

 solution is continuously saturated, there is continuous solution and deposi- 

 tion. Even if no sugar-coated strings were placed in the sugar, after a 

 time it would be found to be coarser grained or to have recrystallized. 

 Thus the constant interchange between a saturated solution and that of an 

 adjacent solid is certain. 



The change occurs under the law by which large crystals grow at the 

 expense of small ones. In order that crystals shall grow in a solvent, it is 

 necessary that the solutions shall be saturated or supersaturated at the 

 immediate place of crystal growth. Since underground there is always a 

 superabundance of many materials as compared with the amount of water, 

 we may suppose that at a moderate depth below the surface, and especially 

 in the smaller spaces, where movement is very slow (see pp. 138-146), the 

 solutions are often saturated. It is well known that the growth of larger 

 crystals at the expense of smaller ones, under conditions of saturation and 

 superabundance of material, g-oes on more rapidly in proportion as the 

 temperature is high and the pressure is great. The principle is taken 

 advantage of in the chemical laboratory in the production, before filtration, of 

 a coarse precipitate by boiling or other means. During the process the finer 

 particles of the precipitate are dissolved and the coarser ones are enlarged 

 at their cost. The growth of the large crystals at the expense of the small 

 ones is due to the fact that the smaller crystals are somewhat more soluble 

 than the larger. The explanation of this change, as given by Ostwald," 

 lies in the "surface tension which exists on the boundary surfaces between 

 solids and liquids, as on those between liquids and gases — the so-called 

 free surfaces of liquids. This tension acts so that the surfaces in question 

 are reduced in size, with the consequent enlargement of individual crystals 

 (the total amount of precipitate remaining practically unaltered), i. e., with 

 the coarsening of the grains." During the change, for a given volume of 

 solid the lessening of the total surface of the crystals, and consequently the 

 lessening of the surface tension, results from the fact that the surfaces are 

 small in proportion as the individuals are large. For a given volume of a 

 substance the surfaces of the crystals are inversely as their diameters. (See 



° Ostwald, W., The scientific foundations of analytical chemistry, translated by George McGowan, 

 Macmillan & Co., London, 1895, p. 22. 



