326 THE JOURNAL OF GEOLOGY. 



velocities and temperatures this difference in viscosity can per- 

 haps be altogether disregarded. The relative power of the 

 atmosphere to sustain a load of fine sediments would, therefore, 

 appear to be no more than, say -gWo"' °f ^^^^ ^^ river water. 

 But to be certain that this estimate shall not be too high, let us 

 make the fraction \ of this value and call it y^\-^. This means 

 that if a cubic foot of water, e. g., in the Mississippi, will hold in 

 suspension 15.48 grams of solid particles ^ then the atmosphere 

 above it can hold in the same manner in a cubic foot yo"J-^ of 

 this quantity, or about .0015 gram. It will be remembered that 

 this is true only for material of a certain coarseness. If it is too 

 coarse, the atmosphere cannot hold it at all ; while if it is very 

 fine, considerably more can no doubt be sustained. In order to 

 ascertain approximately the effect of the variation of the size of 

 the particles on the quantity of materials which can be thus sus- 

 pended in the air, and also to make sure that the above estimate 

 of the total load of sediments which can be sustained is not too 

 high, some simple experiments have been made. These con- 

 sisted in introducing dust of varied degrees of coarseness into a 

 receiver, and then keeping the air in the receiver in constant agi- 

 tation at a velocity of about five miles per hour. A certain 

 quantity of dust would in this manner be kept floating in the cir- 

 culating air, and this quantity was found to vary with the nature 

 of the material introduced. The results may be tabulated as 

 follows : 



Average diaiiieter of Quantity sustained in one cubic foot of air 



particles. agitated to an average velocity 



of J mi. per hour. 

 .08 mm. ... - .020 gram. 



.04 mm. ... - .057 " 



.007 mm, - - - - .118" 



.001 mm. (and below) - - .053 " 



This apparently amply justifies the above estimate as to the 

 quantity of dust which can be sustained in a certain bulk of 

 atmospheric air. It is not supposed that the table gives exact 

 determinations for the different materials, for the conditions of 



' Humphreys and Abbott. 



