684 On the Purification of Water hy means of Iron. 
the native peroxide of manganese, which rests on a substratum 
of sand and gravel, 
rig. 1. — Section of a Filter with Ball-coch complete. 
B. B.iU-cock. 
U. Unfiltered water. 
V. Screw-valve. 
I Spongy iron. 
, S', S". Prepared sand. 
F. Filtered water. 
T. Stop-cock. 
On the large scale there are two wajs of arranging treatment 
by spongy iron. 
According to the earliest method, a double set of filter-beds, 
A and B, Fig. 2, is constructed, one about 4 feet higher than the 
other. 
In the upper bed, A, is deposited a mixture, C, about 3 feet 
deep, of three parts of gravel and one part of spongy iron, 
covered by a layer, D, 6 inches deep, of ordinary filter-sand. 
The lower filter, B, is an ordinary sand-filter, such as is used by 
many water companies. The water to be purified is first 
mechanically filtered in passing through the sand, P, laying 
over the iron and gravel mixture, c, through which it next 
