METHODS OF MECHANICAL ANALYSIS 15 



in motion, the sediment is introduced, and the water current 

 adjusted to the lowest velocity, 0-25 mm. per second; this 

 current is continued till the water ceases to remove any more 

 matter. The operation requires many hours for its com- 

 pletion. The object of the churn is to break up the aggrega- 

 tions of fine particles which are very apt to form. Should 

 any be seen on the sides of the cylinder, the apparatus must 

 be stopped, and the flocks detached with a feather. The 

 water leaving the cylinder is conducted by a tube nearly 

 to the bottom of a tall wide vessel, from the top of which 

 the water runs to waste. The receiving vessel being much 

 wider than the separating cylinder, the upward current of 

 water in it is too slow for any of the solid matter carried 

 into it to escape. 



When no more particles are removed by the current moving 

 025 mm. per second, the receiver is changed, and the velocity 

 of the current increased to 0-5 mm. per second. When the 

 second group of particles has been in this way removed, the 

 velocity of the current is again doubled, and this mode of 

 proceeding is continued till the last separation with a velocity 

 of 64mm. per second is completed. With velocities above 

 4mm. per second the churn may be dispensed with. The 

 work gets more rapid as the higher velocities are reached. 

 When the apparatus is in action day and night the separa- 

 tions will be completed in three or four days. Soft filtered 

 water should be used in all the operations. 



In all modes of separation the greatest difficulty is met 

 with when dealing with the finest constituents. The silt 

 obtained by subsidence from the clay water always carries 

 down clay with it, and can only be freed by rubbing with 

 a caoutchouc pestle and resubsidence from water many times 



