c 



THE ROUNDING OF SAND GRAINS 653 



The effect of surface tension, however, is one aiding wear, since 

 it tends to draw grains together in its effort to force the water to 

 assume the least area permissible under the conditions to which 

 it is subject. Thus viscosity, since it is the most potent of the 

 three factors mentioned, limits the minimum size to which wear 

 takes place. The energy of the particle must overcome the viscos- 

 ity to allow collision. Since the velocity of different grains in 

 water is roughly equivalent, their energy varies directly with the 

 size, the larger grains only having enough power to overcome 

 viscosity. In the case of small grains the water acts as a cushion 

 preventing actual collision, or checking the velocity of contact. 

 To show the action of viscosity in preventing collisions of grains 

 the following experiments were performed. 



Grains were dropped down long glass tubes filled -with liquids 

 of different viscosities, and the action at the meeting of the grains 

 was observed. Grains of different specific gravities 

 were taken so as to overcome the difference in the 

 specific gravities of the liquids. 



Again an experiment was performed (Fig. 3) 

 in which the glycerin was allowed to run from the 

 reservoir C through the tube A A down which the 

 different grains were dropped. The results were 

 practically identical with those above. 



It will be noted that the surface tensions of 

 water and glycerin are nearly the same, but that fig. 3 



the viscosities are in the ratio of eighty to one. 

 In the case of glycerin it was apparently impossible for the grains 

 of small diameter to collide. Whenever a larger grain would over- 

 take a smaller and slower falling one, there was an apparent repul- 

 sion between the two as they were held apart by the viscosity. 

 In small and light grains the repulsion appeared violent so that 

 often a clearing space of a quarter of an inch was shown by grains 

 that apparently were going to collide. As can be seen from the 

 table, in the case of water the protection against collision was 

 much less. Small grains of quartz, less than 1 mm. in diameter 

 showed fairly strong repulsion, but above that size collisions were 

 the rule. Again in the case of alcohol, with a surface tension of 



