Quantities of Foreign Matter on Crystallization. 399 



different dimensions; that is, a deposit takes place more 

 rapidly in a perpendicular than in a lateral direction, the 

 crystal increasing much more in length A C than in width 

 A B in a solution which gives the bevellings ; while in 

 another solution in which the secondary faces are not pro- 

 duced, the crystal increases a little more in width than in 

 length. A complete crystal which had not the secondary 

 faces, was bisected in the direction of its cleavage plane ; one 

 portion was put into the solution which gives the bevellings, 

 fig. 2, and the other into the pure solution, which produces 

 only the single bevel, No. 1. The conditions in other respects 

 were the same, the two vessels being of the same size and 

 material, placed side by side on the same table and exposed for 

 the same number of hours at the same temperature ; and yet 

 next morning the two crystals were completely restored, one 

 with its double and the other with its single bevelled edges, 

 as in figs. 1 and 2 ; but the crystal No. 1 was much wider 

 than No. 2, and No. 2 much longer than No. 1. 



It was supposed that if this mode of increase bore relation 

 to the variation in form, the double bevelment might be 

 excited in the pure solution which does not produce it under 

 normal conditions, by means of some contrivance which would 

 prevent the lateral growth of the crystal, while it encouraged 

 the abnormal growth lengthwise. Accordingly, a single 

 bevelled crystal, fig. 1, had slips of tin foil or metallic paper 

 attached to its lateral facets, while its upper and lower bevels 

 were rubbed down. The crystal thus prepared was put into 

 a pure solution under conditions like those of the crystal in 

 the impure solution. Next day it was found that the crystal 

 had resumed its regular form, the bevels being produced as 

 in No. 2. The experiment was varied in different ways, and 

 measurements were made showing the rate of increase in 

 length and in width of the two varieties of the salt. Hence 

 it appears that foreign matters in solution may, in the act of 

 crystallization, change the relations of increase according to 

 the three dimensions of the salt, an effect which is as much 

 physical as chemical. The relations of the mutual attraction 

 of the saline molecules, according to their different sides, thus 

 become changed by the presence of a minute quantity of 

 foreign matter, and new relations are set up between the 

 number of particles deposited in the unit of time ; thereby 

 allowing or disallowing the existence of certain facets com- 

 patible nevertheless with the primitive form of the crystal. 



Highgate, N., 

 March 31, 1891. 



