25 
During the percolation of water through the layers of 
soil a most effective purification took place. Suspended 
matters were removed, and the matters in solution were 
further oxidised ; and the deeper the water sank into the 
bowels of the earth, the more was it likely to become freer 
from the smaller forms of life which were spoken of as microbes. 
One of the previously-mentioned types of water generally 
served some community as a supply. In some cases the 
water was utilised in its native condition, in others it under- 
went a partial treatment, and yet in others it was subjected 
to a prolonged and intricate treatment before distribution 
to its consumers. The question as to the nature of the treat- 
ment which a water ought to undergo before usage, depended 
almost entirely on the nature and composition of the par- 
ticular supply. Aesthetic tastes preferred a water which was 
clear and sparkling, to a dull-coloured water, such as one 
generally found in a moorland supply ; but, on the other 
hand, the “ spring ” or the calcareous “ deep well ” supplies, 
which were, as a rule, clear and sparkling, were not suitable, 
nor yet economical, for domestic or industrial usage.. Again, 
the heavily-charged river waters were both ugly and extrava- 
gant when they were used without treatment. These remarks, 
said the lecturer, were made on the assumption that the water 
was, of course, primarily safe to use for drinking purposes. 
All the objections he had made could be overcome by a 
system of chemical treatment, followed by artificial filtration, 
or, in some cases, by merely one of these processes. Where 
it was necessary to draw the supply from a polluted source, 
more stringent and more costly methods, of course, had to 
be adoptecl. Even with supplies which were considered to 
be unpolluted, it was oft times more economical to regard it 
as polluted, and to subject the water to the same rigorous 
treatment as would be meted out to a known contaminated 
supply. In certain districts, after a series of epidemic out- 
breaks, experience had persuaded them of their false economy, 
and they now treated their water by processes exhibiting 
all the skill which science could suggest. Accidental contam- 
ination of a supply could occur at any moment, and it was 
only by carefully thought-out treatment that these grave 
dangers might be overcome. 
There were many methods which could be adopted for the 
artificial purification of waters, the most important being 
sedimentation, sand filtration, mechanical filtration, 
aeration with air or ozone, the treatment with chemicals 
and also with bleaching powders. Sterilisation with ultra 
