90 
THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
selves be decomposed under conditions where there is decom¬ 
position going on with a deficiency of oxygen (see ammonia). 
They are always rapidly destroyed, or taken up by aerial 
vegetation, and to a less extent by aquatic, so that this kind 
of evidence may largely disappear from a water. 
Ammonia .—The first thing to decompose in sewage is urea, 
and as this yields free ammonia, the presence of much free 
ammonia in a water is usually a bad sign; it indicates very 
recent contamination, for this ammonia would soon be con¬ 
verted into nitrites and nitrates. There are, however, cases 
where free ammonia does not indicate recent contamination. 
Filtration may indeed increase the amount of free ammonia at 
the expense of albuminoid ammonia.* Again, in underground 
waters the nitrates and nitrites may be themselves decom¬ 
posed in order to yield oxygen for the oxidation of some 
organic matter still remaining, and then the nitrogen may be 
partly evolved as ammonia and partly as free nitrogen, and the 
latter would be lost sight of. 
Protoxide of iron may sometimes reduce nitrates and 
nitrites, taking their oxygen to form a peroxide, and setting 
free ammonia. Sulphates may even be reduced by the prot¬ 
oxide of iron in a similar manner, and so yield sulphuretted 
hydrogen. 
Albuminoid Ammonia .—Many waters contain nitrogenous 
organic matter in a form in which the nitrogen is only slowly 
evolved as ammonia by the action of strong oxidising agents, 
the ammonia so evolved is spoken of as albuminoid ammonia ; 
much of this and little free ammonia would usually indicate 
vegetable contamination. 
Chlorine is often a valuable criterion of contamination in a 
water, because the chlorides introduced by sewage are not 
filtered out, or decomposed, though they may be taken up to 
some extent by plants in the soil, or in a stream. If the 
normal quantity of chlorine in a water from any particular bed 
is known, then the chlorine estimation is useful as indicating, 
if in excess, animal contamination. 
It will have been noticed that all the chemical changes 
just described, as occurring in water, tend to purify it, and 
that the ultimate products of these changes are bodies of 
a simple nature, bodies not liable to further change, and, 
therefore, not injurious in a water. I would only now point 
out that these changes are chiefly effected by filtration and 
aeration, the two operations which are so particularly provided 
for in the plan suggested for filling up the Marlstone. 
[To be continued.) 
* See “Water Analysis,” by J. Alfred Wanklyn, M. K.C.S., pages 98 to 104. 
