THE ROUNDING OF SAND GRAINS 649 



between the grain and water be strong enough to wet the grain, 

 it will sink at once; if adhesion be weak, the grain will remain 

 dry and float. The adhesion between the grain and the water 

 may be entirely destroyed by coating them with oil. The so- 

 called "oil-flotation process" of ore dressing depends to a great 

 extent on this principle. The finely pulverized ore is mixed with 

 a small quantity of oil. The metallic sulphides, such as galena, 

 chalcopyrite, and sphalerite, have strong adhesion for oil, and are 

 readily coated, while the quartz and other gangue remain free, 

 unless an excessive amount of oil is used. When the ore is allowed 

 to slide into the settling tanks, the gangue sinks readily, but the 

 coated sulphides float off. Here it seems that molecular forces 

 cause flotation rather than the decrease in specific gravity due to 

 the combined weight of oil and mineral. As the specific gravity of 

 the oil taken is approximately 0.8, in the case of galena the volume 

 of oil to mineral would have to be in the ratio of 32 to 1, to bring 

 the density of the combined material down to that of water. 1 



Sharp, angular grains float more readily than those of spherical 

 shape. This is due to the fact that the force due to the surface 

 tension increases with an increase in the surface area exposed to 

 it. The more nearly spherical a grain, the smaller the ratio 

 between the surface area and the mass of the grain, and hence 

 the greater the ratio of weight to surface tension. Irregularity 

 of shape increases the ratio of surface to mass, and hence decreases 

 the tendency to break through the surface of the film. 



The power of water to carry material in suspension depends 

 on a number of factors, some of which are: the shape, size, and 

 composition of the particles; the viscosity, composition, and ve- 

 locity of the water; the presence of colloids; the character of the 

 river bottom; the course of the stream, etc. The size of grain 

 carried depends directly on the velocity. The more irregular the 

 shape, the greater will be the resistance encountered in settling. 

 The presence of colloidal substances causes rapid settling. 2 Again 

 there may be a change in the composition of the water causing an 

 interaction with the sediment, such as the precipitation of alumina 



1 Adams, M. and Sc. Press, May 7, 1904, etc. 



- F. W. Clarke, Data of Geochemistry, 430 (Bull. 330, U.S.G.S.). 



