330 GEORGE F. BECKER AND ARTHUR L. DAY 



. The conclusion of these authors seems to be that during growth, 

 material is added only to the upper and lateral faces of the crystal, 

 so that a molecule once added remains at its original level. This 

 was Kopp's contention in opposition to Lavalle, whose conclusions, 

 however, were confirmed by Lehmann and others, including our- 

 selves. This is in fact the root of the matter. If a given increment 

 of the mass after deposition remains at its original level throughout 

 the subsequent growth of the crystal, this exerts no linear force; 

 while if the motion of the particle has a vertical component in 

 consequence of the vertical extension of the lateral faces of the 

 crystal, linear force is exerted. 



On the other hand, if several crystals are immersed, one or more 

 of them being loaded while others are not loaded, the loaded crystals 

 grow only when the concentration of the solution in contact with 

 them exceeds the saturation concentration for each crystal. Pres- 

 sure, of course, increases solubility or raises the point of saturation 

 for most salts. 1 Hence in such circumstances the unloaded crystals, 

 or, more strictly, the less loaded crystals, usually are the only ones 

 to exert lifting power, but in this case, also, growth raises the weight 

 of each crystal. 



Thus Bruhns and Mecklenburg's results with loaded porcelain 

 disks are readily explicable. They experimented with solutions 

 containing many small crystals, some of them weighted, others free. 

 The disks did not rise measurably until the liquid was low and its 

 surface (and consequent rate of evaporation) greatly increased by 

 protruding solid matter, or until the crystals reached from the 

 bottom of the dish to the disks, after which the disks were 

 lifted. 



Repetitions of this operation, extending over a few days, pro- 

 duced aggregate displacements of 5 . o mm. If to this be added our 

 original measurement, twice confirmed in the course of the present 

 control tests, that this linear force, because of the narrow rim 



1 As is well known, if the solution of a solid at constant temperature is attended 

 by a diminution in total volume and a liberation of heat, pressure increases solubility. 

 Such is the case for most crystalline solids including the alums. If the change in 

 volume accompanying solutions is an increase, as in ammonium chloride, pressure 

 decreases solubility. 



