562 
POPULAE SCIENCE EEVIEW. 
forms of animalcules abound there the water may be regarded 
as most impure. As a test example^ sewer- water may be 
taken^ and when similar organisms to those found in sewer- 
water abound, the specimen examined is to be most suspected. 
In making this investigation care should be taken to have 
fair samples of the water. The purest water, if exposed for 
a long time to the air in close vessels, or taken from pipes or 
cisterns which have a long time been exposed to the air, will 
present these organisms. The only fairway of using this test is 
to take a specim en of the water from the mass of the water in 
the river, reservoir, or other source from which the water is 
obtained. The purest water, even distilled water, when kept 
for a length of time exposed to the air, will contain the fila- 
ments of fungi and a variety of animalcules. Under any 
circumstances, living organisms can only be regarded as indi- 
cating the probability that water in which they are found 
contains organic impurities, which by their decomposition 
have afibrded the elements on which the lower plants and 
animals are nourished. That they are more abundant in 
water contaminated with sewage and other organic impurities 
shows the importance of using the microscope as a test of the 
purity of water. 
It is, however, the death of these living organisms and the 
introduction into water of animal and vegetable matters in a 
state of decomposition that renders water impure and danger- 
ous to the health of those who drink it. Both river and 
spring waters are liable to these contaminations. In recent 
times our rivers have been rendered impure by making them 
common sewers of the great towns and cities situated on 
their banks. It is in this way the Thames has become polluted, 
so that the legislature first interfered and required that all 
water companies supplying London should obtain their sup- 
phes above Teddington Lock, beyond the influence of the 
sewage of London, and subsequently passed an Act for the 
carrying of all the sewage of London down to a point in the 
Thames in which it could not be brought by the tide into the 
river as it passed through London. 
It must not, however, be supposed that all organic matter 
produced in rivers or thrown there remains unchanged. No 
sooner is organic matter placed in water than a process of 
oxidation commences, by which the nitrogen of the organic 
matter is converted into nitric acid, and the carbon into car- 
bonic acid. This is effected by the oxygen, which is naturally 
found in all water exposed to the air. As long as there 
is sufficient oxygen in the water to oxidise the organic matter, 
so long the water remains free from those constituents which 
are injurious to health. It too often happens, however, that 
