1892.] DAFERT AND DERBY SEPARATION OF MINERALS. I 29 



5. — After the introduction of the material, the screw- clip on 

 the rubber overflow tube attached at h is opened and all the liquid 

 above / allowed to run off, while any grains that may have remained 

 adhering to the tube above / are washed down by a spirt from the 

 wash-bottle. By operating carefully, that is, slowly, in the intro- 

 duction of the mineral, there is not the slightest danger that the 

 grains in falling into the space ^ will lodge in the tube h and thus be 

 lost. If by chance any do so enter, they may be easily recovered by 

 drawing off the water from h. 



6. — The separation proper may now commence. The cock a^ 

 (fig. 3,) is opened, and b is so regulated as to set the mineral mass 

 slowly in motion. This operation requires some care, as not infre- 

 quently the sand hangs together and only after a little time, moves- 

 as separate grains. A slight tapping of the apparatus is often of 

 good effect at this point of the operation. The current should be 

 so regulated as to raise the column of moving sand to near the point 

 /, taking care, however, not to allow it to enter the tube/;. In a short 

 time the lightest and heaviest grains will be seen to be concentrated 

 at the highest and lowest parts of the column respectively, while in 

 the median portion of the column are the minerals of intermediate 

 specific gravity mixed with the larger grains of the lighter and the 

 smaller of the heavier minerals. 



7. — The withdrawal of the sand in as many successive portions 

 as may be desired, commencing with the lightest, is effected by lower- 

 ing the tube b until its upper opening reaches the point at which a 

 separation is desirable. Care should be taken to do this so gradually 

 as not to lower the level of the liquid at / whereby the force of the 

 current would be influenced. As soon as the upper end of the tube 

 reaches the column of moving sand, the grains that pass its level 

 sink quietly and regularly into it, without in any way influencing the 

 separation still going on at the lower levels. By opening the spring- 

 clip w, the contents of the tube b can be drawn off at any time. This 

 operation should also be effected with care so as not to bring the level 

 of the liquid column below / and so alter the pressure. It is clear 

 that by successive lowerings of the tube b, the entire contents of the 

 apparatus can be drawn off in as many portions as may be desired. 

 A good plan is to draw off liquid enough from b to bring the sand 

 accumulated in it into the free portion below the cork where the sep- 

 arated part can be readily examined with a lens and the proper level 

 for the tube, or the proper time for drawing off, be thus determined 

 more accurately than by watching the moving grains in the apparatus.. 



9, Proc. Roch. Acad, of Sc, Vol. 2, Feb., 1893. 



