292 Purification of Water. 
precipitates, taking with them alumina and other impurities 
which discolour the water. But this process has the draw- 
back that but a slight excess of alum will form sulphates of 
lime, which, remaining in solution, causes hardness. 
The mechanical difficulties of constructing an effective 
and lasting filter for domestic use are many and have caused 
many failures. The maker must not only be thoroughly 
acquainted with the nature of the water he has to deal 
with, but le must also well consider the local influences. 
He may have excess of pressure in the supply or want of 
it. Moreover, he has to study the convenience of his 
customers, if he wishes for complete success. 
A water which is sufficiently pure in itself, and only 
subject to accidental and occasional contamination offers no 
difficulty. ‘The case however is different if, for imstance, 
it holds lead and excess of organic matter in solution. 
Here he has not only to remove this impurity, but he runs 
the risk of getting flat water, on account of its being invari- 
ably of a soft nature in this case. With due regard to the 
porosity of the filtermg medium, in accordance with the 
pressure of the supply, the filter maker will principally choose 
animal charcoal corrugated with steel. The lead coming in 
contact with animal charcoal precipitates mstantaneously, 
and the joint action of the media employed will reduce the 
organic matter to a minimum, causing at the same time the 
water to be bright and sparkling. 
To describe the mechanical arrangements of apparatuses 
for various purposes would be out of place. I may merely 
mention that the small filter which I have brought here has 
only been constructed for conveniently testing the filtering 
medium for given purposes. 
Much as I prefer animal charcoal to other materials in 
many cases, it would be folly to overrate its qualities by 
adopting it as a cure for every impurity which may be found 
in water. 
In 1860, in a ier which I wrote to The Times, public 
attention was called to the fact that animal charcoal, | 
when judiciously employed, will remove solution of lead 
from water. It was amusing to see how nearly all the 
London filter makers at once adopted the cry for their adver- 
tisements :—‘‘ No more lead in water,” and commenced using 
nothing but animal charcoal for their filters, thinking that 
this was all that could be desired. Of course their ignorance 
led them astray again, because the effect of animal charcoal 
