1866.] with the supply of water to Calcutta. 223 
possessed of any such striking properties as to aid us much, as they 
are mostly of a neutral nature without active chemical or physical 
characteristics. Nitrogenous bodies, however, yield products more 
readily recognised, and as it is this class of substances which are most 
likely to possess properties injuriously affecting the animal economy, 
their detection is also the most important. 
The ultimate products of the decomposition of nitrogenous organic 
substances “are, in addition to water and carbonic acid, also. ammonia, 
and where excess of oxygen is present, nitric acid. But there are also 
numerous intermediate products, and these are often characterised by 
offensive smells which give a certain character to the putrefaction of 
animal substances, different from that yielded by the fermentation or 
corruption of vegetable bodies. The smell or flavour then of a water 
is a very good test of its purity, though it indicates rather the stage 
of decomposition in which its organic matter exists than the amount 
of organic matter present. And in connection with this I may 
mention the test of keeping the water and observing the changes 
which take place in it, the production of animalcules or of aquatic 
vegetation. Now Ihave kept samples of water taken from the river 
at all seasons for many months. Those taken during the cold and 
hot seasons settled easily and suffered very little further change; at 
the most a little greenish deposit at the bottom of the bottle formed, 
which is the case, however, with ordinary distilled water. It was very 
different, however, with the water of the rainy season. Some water 
taken from the river on 3lst August, 1865, was kept for about two 
or three weeks, then syphoned off the deposited mud into other clean 
stoppered bottles in which it remained, the bottles being closed for 
about four months, when the bottles were found to have their sides 
covered with abundant green branching vegetation: the water was 
again syphoned off quite clear to other clean bottles and kept for about 
six months longer, when the same appearances were observed, though 
to a much smaller extent. There was abundant proof in this case 
of the presence of organic matter, probably both in the form of living 
germs and of chemical compounds dissolved in the water. The water 
taken during the hot season may have contained as much: possibly 
the presence of the excess of saline matter may prevent such develop- 
ment, but I am not prepared to give an opinion on the subject. The 
28 
