S. Taber— Growth of Crystals. 543 



paper unci then with a piece of the metal screen. The holes 

 in the screen were 1/5 millimeters in diameter and were 

 spaced 0*75 millimeter apart. The solution was .allowed to 

 cool down over night and the following morning the crystal 

 was removed, sectioned vertically and carefully examined. 

 It increased in height from 4*69 to 5*15 millimeters through 

 the addition of material to the lower surface, and the hollow 

 in the original crystal was almost entirely filled. Moreover, 

 the crystal had grown downward in places through the pores 

 of the filter paper, as there were several crystals, oriented 

 parallel to the original crystal, attached to the under side of 

 the lower metal screen. There was no deposition of new 

 material on the upper face of the crystal and no growth 

 through the paper such as was observed underneath, although 

 the crystal grew outward until beyond the limits of the 

 overlying filter paper and screen, and then upward and also 

 inward until the new growth overlapped the screen on top. 

 The pressure of the filter paper and screen, weighing less than 

 0*5 gram, was sufficient to prevent any growth on the upper 

 surface of the crystal or in the pores of the filter paper. 

 Growth continued downward through the pores of the lower 

 filter paper because the concentration of the solution increased 

 in that direction. 



In another set of experiments, growing crystals of alum, 

 copper sulphate and potassium sulphate were subjected to 

 lateral pressure between pairs of vertical glass plates while in 

 all other directions they were in free contact with supersatu- 

 rated solutions. These crystals grew in every direction, except 

 that in which the pressure was applied, forming flat tabular 

 crystals. They could not overcome the external pressure 

 resisting growth, because there was no provision for supersat- 

 urating the solution with respect to the faces that were under 

 pressure and that were thereby rendered more soluble. 



A consideration of the results obtained in the foregoing 

 experiments leads to the following conclusions: 



(1) A crystal will grow in a direction in which external 

 forces oppose growth, if the surface on which the forces are 

 acting is in contact w T ith a solution that is supersaturated with 

 respect to it; and, if the growing crystal is composed of a 

 substance the solubility of which increases with pressure, then 

 for any increase in the forces opposing growth a corresponding 

 increase in the concentration of the solution is necessary. 



(2) If a crystal of such a substance is subjected to a greater 

 pressure in one direction than in others (as for example when 

 a weight is placed on a crystal), the surfaces that are under 

 the lesser pressure will tend to limit the concentration of the 

 solution and prevent it from becoming supersaturated with 



