On the Purification of Water by means of Iron. 685 
passes, and becomes impregnated with iron ; it is then conveyed 
into the lower filter by the pipe, E, and falls in such manner as 
to be as much mixed with the air as possible. The iron which 
had been dissolved is deposited, chiefly in the form of the red 
oxide, and is, together with the impurities rendered insoluble, 
separated by the sand-filter. The purified water contains no 
trace of iron. 
Fig 2. — Section of a douhle set of Filter-beds. 
The filter-area required for this process varies, with the 
quality of the water, from 50 to 150 gallons per square foot of 
surface per twenty-four hours for each filter. The attention 
necessary is little more than that which has to be bestowed on 
all sand-filters, namely, the periodical scraping off and washing 
of the top-layer about one-half inch deep of sand. The mixture 
of iron and gravel, especially with very bad water, has occasion- 
ally to be uncovered and broken up, and the layer nearest the 
upper surface washed. 
An instance of the application on a large scale is afforded by 
the Antwerp Water Works, where in 1880 Professor Bischof's 
method was adopted, and has continued to work with very 
satisfactory results ever since. After three years' continuous 
action there is no sensible diminution of the quantity of iron. 
The waters of the Nethe, which are yellow and turbid, are ren- 
dered beautifully bright, clear, and palatable, and, according to 
a recent analysis, the free ammonia is reduced from 0"032 grains 
per gallon to 0 004 grains, or to about one-eighth, and the 
albuminoid ammonia from 0'013 grains per gallon to O'OOG, or 
to less than one-half. Full particulars of this application will 
be found in a paper by me, which was read before the Institu- 
tion of Civil Engineers on January 16th, 1883 (' Proceedings' 
Inst. C.E., vol. Ixxii.). 
Although the system adopted at Antwerp has proved so suc- 
cessful, the cost of the application was very great, on account 
of the large quantity of spongy iron, amounting to 900 tons, 
required to charge the filters, the large area of land covered by 
