1885*] Flow, When, and Wheve is Sewage Injuvious ? 271 
Tlic thiid and fourth states in which nitrogen occurs in 
sewage are as ammonia and as nitric and nitrous acids, the 
two latter either in combination with the ammonia or’ the 
potash and soda, forming nitrates and nitrites. 
Ammonia with its salts, and the nitrates and nitrites, even 
in the laigest propoition in which they can occur in sewage, 
are harmless in themselves ; but, like organic nitrogen, they 
afford nourishment for microbia. Moreover, these three 
forms, organic nitrogen, ammonia, and the nitrates, are con- 
stantly passing and repassing into each other. Growing 
fungi, ^ and other plants, convert ammonia and the nitrates 
and nitrites into organic compounds of oxygen. The only way 
to make any water absolutely incapable of nourishing low 
forms of life, is to keep it free from combined nitrogen in every 
shape and state. But with waters exposed to the air this, in 
the absolute sense of the words, is impracticable. 
Phosphoric acid is another ingredient introduced into 
water by pollution with the excrements of animals, or with 
decomposing organic matter. It is not an absolute proof of 
the presence of such pollution, since natural waters, on 
careful examination, may be found to contain it in very 
small quantity. 
It is, in itself, not merely harmless, but in most cases, 
doubtless, beneficial. Yet here, again, it may be an in- 
direct source of danger by favouring the multiplication of 
microbia. 
Much the same may be said of potash. It is noteworthy 
that the three constituents, combined nitrogen, phosphoric 
acid, and potash, which are most valuable on the land as 
being necessary to the growth of our food-crops, should be 
most dangerous in the water as fostering the growth of 
disease-germs. One and the same kind of matter, accord- 
ingly as it is in the right or the wrong place, becomes the 
source of life or of death. 
Common salt (sodium chloride) is more largely present in 
sewage than in normal natural waters. It is, of course, 
harmless to human life, even in greater proportions than it 
is ever known to occur in sewage. But*it tells a tale of ani- 
mal pollution. If in water there is more than 1 grain of 
chlorine per gallon (= nearly if grain common salt) we have 
strong reason to suspeCt that sewage, or, at least, the blood 
or the urine of animals, has found its way into the river 
in quantity. 
There are, of course, exceptions when salt springs or beds 
of saliferous minerals occur in the district, or if aluminium, 
iron, tin, &c., chlorides have been introduced by industrial 
