682 On the Purification of Water hj means of Iron. 
up the subject, that anything really practical was done. Since 
that period, however, a great development of the use of iron has 
taken place, and it has now reached a position of such hygienic 
importance, that a notice in the pages of this ' Journal ' cannot 
fail to be both interesting and useful. 
In an excellent article on the composition and properties 
of drinking-water, and of water used for general purposes,* 
Dr. Voelcker has fully explained the conditions Avhich render 
water fit for the arts and for dietetic purposes ; and he alludes in 
that article to those important properties of spongy iron which 
Professor Bischof has laboured with unwearied constancy to- 
adapt to the purification of water. 
First, Professor Bischof had to provide the metal in a suitable 
form, in sufficient abundance, and at a low price for use in 
filters. This he accomplished by improving the process of 
reducing the " purple ore " of commerce, the refuse of the 
roasting of iron pyrites, by means of coal and coke, regulating 
the temperature of the furnace so as to allow the particles of 
reduced iron to stick together very loosely and produce a highly 
porous substance, to which the name of " spongy iron " has- 
been given. Next, he turned his attention to the application of 
iron to domestic filters; and it is important to note that the 
principle on which iron acts upon impure water is totally 
different from that of any other filtering substance. It does not,, 
in fact, act as a filter at all, but it is slowly dissolved by the 
water, forming carbonates and low hydrated oxides, which pass- 
into a higher state of oxidation at the expense of free oxygen 
in the water, or of oxygen taken up from the air by the water 
after its passage through spongy iron. The precise action is 
not well understood, but it depends, speaking broadly, upon the 
instability of the lower oxides of iron, and upon the absorption 
of the free oxygen in water. 
The tendency to combine with free oxygen is probably one 
of the causes of the destruction of animal and vegetable life 
which undoubtedly takes place in water purified by iron. It 
is well known to those interested in aquaria, that water which 
has been in contact with metals, and especially iron, is not 
suited to many kinds of fish ; hence the pumps used have glass 
barrels, and the pipes and other parts exposed to the water 
are made of ebonite. Again, to agriculturists the presence, 
in the soil, of protosalts of iron, such as are formed by the 
action of ordinary water upon spongy iron, is a sure sign of 
sterility of the soil. 
In combining with the free carbonic acid in the water, iron 
* ' Jo-jmai; I!. A. S. E. 1875, vol. xi. p. 127. 
