WATER SUPPLY AND PUBLIC HEALTH. 
37 
Wells on bare chalk give water which always contains nitrates, 
but from which ammonia is frequently absent. In fact, water 
from chalk when under London clay appears somewhat to resemble 
the water from the lowest Tertiary strata, which lie between the 
chalk and London clay ; probably the chalk in such situations is 
partly fed by water draining downwards from the overlying sands. 
Chlorine exists in water chiefly as common salt (chloride of 
sodium). Water which has been much polluted by sewage 
always contains this substance in considerable quantity ; it is 
therefore a guide in reading the history of waters. But sewage 
is not the only source of chlorine. The rain and the air of 
maritime districts are often charged with sea spray, and 
during heavy storms salt has thus been carried far inland. 
After the severe storm of Jan. 1839 salt was found on the trees 
near Huddersfield, sixty miles from the sea. In Nov. 1703 salt 
was carried into Sussex, more than ten miles from the coast, 
making the leaves and branches quite white. Salt will also 
occur in water after infiltration through strata naturally con- 
taining it. 
Much discussion has arisen as to the phrase a previous 
sewage or animal contamination,” in which Dr. Frankland sums 
up the history and quality of various waters ; some chemists 
objecting to the phrase, as sensational or wholly delusive. The 
meaning of it is this': — An estimate is made of the total amount 
of combined nitrogen contained in solution in 100,000 parts of 
average London sewage. This amount varies at different 
times; in 1869 it was 7*06 parts, in 1857 it was 8*363. For 
simplicity the number 10 has been assumed. But rain-water 
always contains nitrogen in these forms ; numerous analyses 
give a mean of *032 per 100,000 parts as the amount of 
nitrogen present in rain water as ammonia, nitrates, or nitrites. 
“ After this number (*032) has been subtracted from the 
amount of nitrogen, in the forms of nitrates, nitrites, and am- 
monia, found in 100,000 parts of a potable water, the re- 
mainder, if any, represents the nitrogen derived from oxidized 
animal matters with which the water has been in contact. 
Thus a sample of water which contains, in the form of nitrates, 
nitrites, and ammonia, *326 part of nitrogen in 100,000 parts, 
has obtained *326 —*032 — *294 part of that nitrogen from 
animal matters. Now this last amount of combined nitrogen 
is assumed to be contained in 2,940 parts of average London 
sewage, and hence such a sample of water is said to exhibit 
2,940 parts of previous sewage or animal contamination in 
100,000 parts; or in other words, 100,000 lbs. of the water 
contain the mineral residue of an amount of animal organic 
matter equal to that found in 2,900 lbs. of average London 
sewage.” * 
* River Pollution Commission, Sixth Report, p. 14. 
