26 
violet light, and submitting the water to periods of storage, 
were amongst the recent developments. In most cases two 
or three of these processes were applied to one water, with 
the idea of removing as much of the undesirable matter as 
possible, producing a water of the required composition, 
and rendering the water as free as possible, from any 
chance of containing those organisms which gave rise to 
specific diseases. Sedimentation basins were used for 
removing the grosser suspended impurities, and, at the 
same time, the water was slightly aerated, and, if need 
be, might at this stage be treated partially, so as to 
remedy any injurious feature. Sand filters were used 
with the object of removing as much of the suspended 
matter as possible, so that the water issuing from 
them contained as few living forms as possible, and that the 
organic matter in solution should be liable to easy oxidisa- 
tion, and eventually removed altogether. When the organic 
matter had been perfectly removed, the water contained no 
appreciable quantity' of those compounds which were capable 
of supporting microbic and the lower forms of animal life ; 
and so, should any accidental contamination occur at this 
stage, the organisms so introduced would have a less chance 
of propagating, there being only food for them such as was 
introduced along with them in the actual pollution. 
Mechanical filters were used, with much the same object 
as the slow sand filters, but it was general nowadays to 
accomplish a partial chemical treatment of the water at 
the same time. Aeration with air was a pure attempt 
at oxidisation, so as to reduce the amount of soluble 
organic matter, and consequently make the water such that 
it was difficult for animal life to flourish in it. Oxidisation 
was an intensification of the simple aeration treatment, but 
was probably more efficient. Chemical treatment with 
chlorine or chlorine compounds was directly a poisoning or 
a killing off of the smaller forms of life, and eventually an 
oxidisation due to the nascent oxygen liberated. This 
method had its objections, in that the chlorine or chlorine 
compounds produced in the water were nauseous in flavour. 
One of the most recent attempts at sterilisation of a water 
supply, after treatment, was by making use of ultra-violet 
light. These particular light rays were deadly in their 
effect on the smaller forms of life, and in order to dispense 
with these germs the water containing them was made to 
pass, in thin layers, over an area upon which these light ray's 
were focussed. The advantage of this method was that 
nothing was added to, or abstracted from, the water, and 
