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It becomes, therefore, doubly necessary that we should be able 
to trace with certainty the flow of water from its source until 
delivered at our houses, in order to feel sure that it runs no risk 
of contamination by the way, for purity of source and surround- 
ings of the water supply for domestic use must be regarded as 
paramount to anything that chemistry may be expected to reveal. 
We will now deal with the water as delivered in our houses. 
Supposing it to be advisable to soften the water—and we are 
willing to incur the expense—there are several systems by which 
‘we can have all the water for domestic purposes softened and 
filtered in our own homes. The Porter Clark arrangement and 
that of Maignen both succeed in effecting this improvement. 
The fittings, however, are somewhat troublesome to arrange, and 
a certain amount of supervision and attention is necessary to 
ensure the efficacy of the working. 
It is advisable in all cases to filter the water supplied by the 
mains, when required for drinking ; this is doubly necessary in the 
case of an intermittent supply when the water is stored in cisterns. 
These cisterns are a fruitful source of impurity, and are as a rule 
placed in the most out-of-the-way places, as if on purpose to 
increase the difficulty of cleansing them; and instead of one main 
cistern, in many cases there are several subsidiary ones. 
This leads to the subject of filters. 
Filters seem to have been in use from a very early period, as 
they were known to have existed both amongst the Japanese and 
Egyptians. These filters were made of stone or unglazed earthen- 
ware. Filters of dripstone have been commonly used both in the 
East and West Indies. 
The first improvement on the stone filters seems to have been 
the use of charcoal and sand as a filtering medium, and various 
adaptations of these substances have formed the materials of which 
filters have been constructed up to a recent time. 
Such filters are now quite out of date, as they have proved 
mere traps for collecting impurities, the filtering medium being 
