542 8. Taber — Growth of Crystals. 



and lower surfaces of the crystals used are crystallographically 

 the same, and therefore the forces controlling the development 

 of crystal faces must be the same at the top as at the bottom. 

 The pressure on the upper surface of a crystal is always less 

 than that applied to the base, for the latter must also support 

 the weight of the crystal, and therefore, all other things being 

 equal, a crystal should grow through additions at the top 

 rather than at the bottom. That this did not take place in the 

 foregoing experiments leads inevitably to the conclusion that 

 the upward pressure developed by the growing crystal must be 

 explained by the presence of a solution, in contact with the 

 base, having a greater degree of concentration than the solution 

 in contact with the other surfaces of the crystal. 



It is a well-known fact that when solutions of a salt, such as 

 alum, are subjected to concentration, the denser and therefore 

 more concentrated portions of the solution tend to settle to the 

 bottom of the container. The roughly pyramidal form of the 

 larger alum crystals has already been commented on, and this 

 shape is unquestionably due to the more rapid growth of the 

 lower parts of the crystal where they are in contact with the 

 more concentrated layers of the solution. 



If a crystal of alum is suspended just below the surface of a 

 saturated solution in a large beaker and another crystal is 

 placed on the bottom of the beaker, the latter crystal will 

 grow in size at the expense of the former, which will go slowly 

 into solution. 



A crystal of alum 3 milligrams in weight was placed on 

 a small glass plate suspended 2 centimeters above the bottom 

 of the crystallizing dish containing a warm concentrated 

 solution, while a similar crystal was placed on the bottom 

 of another dish containing a solution of the same concentration 

 and temperature. Both were allowed to cool down over 

 night under exactly the same conditions. When the crystals 

 were examined the next morning it was found that the 

 crystal resting on the suspended glass plate had been raised 

 through a distance of only 1*1 millimeters, while the crystal 

 that had grown at the bottom of the crystallizing dish had 

 been raised 2*4 millimeters. 



The experiment described below illustrates nicely the dif- 

 ference in the growth of a crystal on its upper and lower 

 surfaces because of the greater concentration in the lower 

 layers of the solution. A perforated metal screen covered 

 with a piece of filter paper was supported on glass rods so 

 that it rested 3 millimeters above the bottom of a crystallizing 

 dish containing a warm concentrated solution of alum. A 

 ci^stal of alum colored with chrome alum was placed on top 

 of the filter paper and covered first with a small piece of filter 



