ON WATER-FILTERS. 
35 
matter in water to do harm, and jet these evidences not be attain- 
able, or, at any rate, not to an extent that can be relied upon. 
However, the presence of a very little organic matter in a 
water the history of which shows its impurification with sewage 
to be improbable, does not make it impure for drinking 
purposes. For not only is it difficult, if not impossible, to ob- 
tain water altogether free from organic matter, but experience 
shows that water whose antecedents can pass unchallenged, such 
as the water of the lakes, may contain a small quantity with- 
out harm. 
One character of organic matter by which its power for evil 
may be judged of, is its liability to spontaneous decomposition — 
in its liability to putrefy and decay. The process of putre- 
fying is indirectly a manifestation of the extent of the power 
stored up in these matters when they were part of a living or- 
ganism, and, what is of greater import to us at present, an evi- 
dence of the power they have to do something — it may be good, 
but more likely evil — to our bodies. The test of the extent to 
which the bodies in water can putrefy or otherwise exhibit ac- 
tive powers is their capacity of combining with oxygen. Matters 
which have already combined with oxygen as far as possible, and 
matters which show little readiness to combine with it at all (at 
least those likely to occur in water), are both non-putrescible and 
inactive apparently when swallowed. Hence the oxidisability 
of a drinking-water becomes an important indication of the 
likelihood of the organic matter proving injurious. The degree 
of oxidisability of a water is estimated by means of the action 
upon it of permanganate of potash , a substance readily yielding 
oxygen. But the absence of any readily oxidisable matter in a 
water is shown in another way, which it is of interest to consider 
here. 
We have already mentioned that a pure drinking-water may 
contain oxygen and nitrogen besides other gases. These gases 
in water are derived from the atmosphere, and it is found that, 
in consequence of the superior solubility of oxygen in water, 
twice as much is dissolved of it as of nitrogen although nitrogen 
is four times more plentiful in the air. Now, if readily oxidis- 
able substances are in the water, they become oxidised at the ex- 
pense of this oxygen dissolved, and the result is that such water 
contains less oxygen (if any) than twice as much as the nitrogen 
in it. If the water contains less than a third of oxygen of 
what it does of nitrogen, it may be set down as imperfectly 
aerated, at least so says Dr. W. Allen Miller, and as therefore 
containing readily oxidisable matters, that is, matters mostly 
dangerous to health. 
Of all the qualities which a water must be free from in order 
that it can be looked upon as a pure drinking-water, there is one 
VOL. vi. — NO. XXII. E 
