METHODS OP ANALYSIS 57 



lbs. of ore. Proceed as with the percolators, but allow the solution 

 to pass through filiform zinc in two or more glass or porcelain 

 cells. Note the precipitation and analyze the solutions on com- 

 pletion of the tests. Be careful that the solutions are aerated 

 before returning them to the leaching vats, for, as the solutions 

 in percolating through a charge of ore are gradually deprived of 

 their oxygen, it is found that the lowest layers of ore, those nearest 

 the filter, do not extract as well as those layers above. This can 

 be remedied by running on an occasional aerated solution from 

 below. This must be performed slowly, to avoid forming holes 

 or channels in the charge. In this way solutions rich in oxygen 

 are brought in contact with the bottom portions of the charge, 

 and a more even extraction thus obtained. 



