THE MIDDLE LIAS OF NORTHAMPTONSHIKE. 
89 
not advisable to speak very decisively, but it does seem very 
unlikely that germs which find such a suitable habitat—for 
themselves—in the hot fluids of the body would thrive in cold 
water, and if not they must ultimately die out, and become 
burnt up. Fortunately with regard to the second point more 
decided evidence is available, for Dr. Percy E. Frankland has 
shown that filtration not only separates distinguishable solid 
particles, but also reduces the number of micro-organisms, the 
reduction extending to entire elimination, according to the 
number of organisms present, and the condition or extent of 
the filter. Spongy iron or coke are the most effective 
materials at present known, but it is important to know that 
simple agitation of the water with coke, charcoal, or chalk will 
effect the precipitation of these organisms, and a like result 
will be obtained when precipitating the lime in water by Dr. 
Clark’s process of softening. The microbes are not killed by 
this latter method, and they will re-ascend into the water if it 
is left long in contact with the sediment. 
Records of contamination. —Oxygen dissolved in water is 
more active chemically than the same gas in the air, and 
when dead organic matter is discharged into water the con- - 
stituents are rapidly changed, so that to detect contamination 
the products of oxidation and decomposition have to be 
looked for. 
The carbon of the organic matter is oxidised to carbonic- 
acid gas, which remains dissolved in the water, and cannot 
be identified in an aerated water, since such water would 
normally contain it. 
The hydroyen is oxidised to water, which, of course, 
cannot be distinguished in the same medium. 
The nitrogen is partly converted into ammonia , and partly 
into nitrous and nitric acids, which latter substances unite with 
the bases always present in water which has passed through 
or over the ground, forming nitrites and iiitrates; and since 
rain-water only contains minute traces of the above-named 
acids, the presence of determinable quantities of these 
nitrites and nitrates is looked upon as a record of previous 
contamination, they are considered to be the harmless 
skeletons of organic bodies. In the chemical changes just 
referred to we have an imitation of the production of nitre. 
Nitrates and nitrites. —It is estimated* that 97 per cent, of 
the combined nitrogen of London sewage is converted into 
nitrates during slow percolation through a stratum of gravelly 
soil only five feet thick. Nitrates and nitrites may them- 
* Sixth Report of Rivers Pollution Commission. 
