9 2 



SOILS. 



and which eliminates counter-currents, continually disintegrates 

 these compound particles, and thus enables the entire quantity 

 of the sediment corresponding to the prevailing current-velo- 

 city to pass off with a comparatively slight expenditure of time 

 on the part of the operator (see figure 7). A wire screen in- 

 terposed between the churn and cylindrical glass tube prevents 

 communication of the whirling motion to the column. As the 

 apparatus works automatically, the analyst has only to observe 

 from time to time whether or not the turbidity near the top of 

 the tube has disappeared ; and as the sediment accumulates at 

 the bottom of the tall receiver bottle, 1 no harm is done if the 

 attendant should neglect to change the velocity in time, except 

 ttyat water will run to waste. 



The conical relay glass below the churn serves to retain the 

 coarser grades of sediments which are not concerned in the 

 velocities employed in the elutriator tube, and thus prevents 

 injurious attrition. But these sediments can at any time be 

 stirred up by the incoming current and brought into the wash- 

 ing tube if desired. In the same manner the passing-off of 

 the finer sediments can be materially accelerated by running 

 off rapidly about two-thirds of the turbid column of water 

 every twenty minutes. 



It should be fully understood that prior to attempting such 

 separation, the " colloidal clay " must first be removed by the 

 subsidence or centrifugal method, since otherwise much larger 

 grain-sizes may be carried off at a given velocity. 



Voder's Centrifugal Elutriator. A very ingenious instru- 

 ment which combines the elutriation and sedimentation pro- 

 cesses into one, has been devised by P. A. Yoder, of the Utah 

 Expt. Station. The elutriator bottle is placed in a centrifuge 

 driven by an electric motor; it is closed by a glass stopper 

 carrying a delivery tube to a short distance above the bottom 

 of the elutriator bottle, as well as an outflow tube ending at the 

 base of the stopper; the latter also carries a funnel coinciding 

 with the center of rotation. Into this funnel flows gradually 



1 The figure given of this elutriator in Bulletin N T o. 24, on physical soil analysis, 

 published by the U. S. Bureau of Soils, shows as the receiver a bottle entirely too 

 low to insure the complete retention of the sediments by settling. The receiving 

 bottle should not be less than twelve inches high and five inches wide. 



