46 SCIENCE PROGRESS 



4. Preparation of Material 



In the case of sands, the separation can usually be made 

 without crushing, as the grains are generally small and of a 

 simple character; but it is often convenient to sift, in order, 

 as far as it is practicable, to secure uniformity of size among 

 the grains. 



With rocks, the degree of crushing required depends upon 

 the texture, but as a rule the diameter of the grains obtained 

 should not exceed 0*25 mm. The crushing should be done so 

 as to secure a minimum of powder, especially in quantitative 

 operations. This is effected by sifting at frequent intervals 

 during the crushing process. The powder should be washed 

 off, and, if necessary, treated separately in the wet way according 

 to Pebal's suggestion. In any case the treatment of the powder 

 usually gives unsatisfactory results, although for quantitative 

 purposes it obviously cannot be neglected. On this point it is 

 important to note that some minerals powder more readily than 

 others, hence the proportion of any mineral present in the 

 powdered portion will differ from that of the same mineral in 

 the coarser portion of the crushings. 



For the purposes of isolation, or in merely qualitative 

 separation, the powder may be washed off and rejected after a 

 cursory examination with the microscope. 



5. Separation of the Minerals 



Magnetite, if present, should first be extracted by a weak 

 permanent magnet — an operation which is facilitated by covering 

 the poles with a movable paper cap. The portion thus extracted 

 should be carefully examined, as it may contain pyrrhotite and 

 even haematite, both of which are very strongly magnetic. 

 Ilmenite is not extracted by a weak magnet, except in the finest 

 powder, and then only to a small extent. 



The remainder of the grains are placed on cardboard, and, 

 assuming the current to be switched on, are presented to the 

 poles of the electromagnet. The cardboard is then removed, the 

 current shut off, and the adhering grains allowed to drop into 

 a cardboard box placed under the poles. This is continued till 

 no more grains adhere to the poles. A certain amount of non- 



