356 Dynamic Theory. 



The polarity of the first molecules forming a nucleus of crystalliza- 

 tion, determines their position with reference to each other, and as a 

 crystal is built up, the shape of it tends to constantly remain the same 

 as the initial nucleus of molecules. If, in a solution of nitre and sul- 

 phate of soda, a crystal of nitre be dropped, all the dissolved nitre crys- 

 tallizes, the sulphate remaining in solution. But if a crystal of sul- 

 phate of soda be dropped into the solution, instead of the crystal of 

 nitre, all the sulphate of soda will become crystallized, leaving the 

 nitre still dissolved. 



" If a portion of a crystal be broken off or filed or dissolved away, 

 and if then the mutilated crystal be replaced in a solution of the same 

 salt or of an isomorphous one, the lost part will be replaced, the crystal 

 will be enlarged, new crystallized matter will be formed on every sur- 

 face ; but the quantity formed on the injured part will be greater than 

 that formed at any other part and repair will be more active than mere 

 growth till the proper form of the crystal is regained. Then when the 

 repair is complete, growth alone will go on and each part of the crystal 

 if it remain in the same solution will increase in due proportion with the 

 rest." A crystal once formed in a solution ( or at least every face of 



FIG. 154. Alum Crystals. 



a. Ideal form obtained when the crystal is 

 C9nstantly turned during its growth so as to 

 give all sides an equal chance. 



b. Distorted by growing in contact with the 

 side of the vessel. 



any set of its faces ) would continue 

 to grow equally by the deposit of the 

 same amount of material on every 

 side But in reality it rarely does, 

 FIG. 154. because the formation of the crystal 



changes the specific gravity of the solution and that changes the 

 currents in the crystal, so that the deposits on the different faces 

 become unequal. This is called distortion, and it is also brought 

 about by the crystal becoming attached to another crystal or to the 

 side of the vessel. " When new material is very quickly presented 

 to a growing crystal it is often noticed that the acuter solid angles grow 

 very rapidly, shooting out long needle-like points, and often other 

 acicular points will start from along the first, and thus fern-like forms 

 are produced ; all such growths are called crystalline skeletons ; when 

 the rate of deposit becomes less, the needles almost cease to grow in the 

 direction of their length, but increase continually in breadth and thick- 

 ness until they touch each other and the crystal returns to its original 

 appearance, though often containing a great number of cavities filled 

 with the mother-liquor, and these cavities will take the general form of 



