222 Mr, Waldie’s investiyations connectcd [No. 3, 
remainder of the year, vegetable and animal matter of every kind is 
deposited in or upon the soil in all stages of decomposition. The 
amount of drainage is small and the flow of water gentle: the water 
carried thus to the river is comparatively pure, and that from the 
sources of the streams is from places bare of vegetation and part of 
it from melting snow. But when the rains come, they wash off all the 
accumulated products of decomposition of vegetable and animal sub- 
stances in the state both of solution and suspension, of which the 
appearance alone of the water and its flavour give ample evidence. 
The increased proportion, it is true, is counteracted by the largely 
increased quantity of the water which dilutes it; for if, instead of 
looking to the proportion of organic matter to the water, we look to 
its amount in proportion to the inorganic or mineral saline matter, 
then in the rainy season the excessive proportion of organic matter is 
rendered much more evident. After the rains the mud subsides, 
which is favourable to the purification of the water, and the atmo- 
spheric oxygen contained in solution in the water, as it is in natural 
waters generally, acts upon the organic matter in solution, oxidizing 
and destroying it. And as heat in general materially increases the 
energy of chemical action, there can be little doubt that this purifying 
influence goes on more rapidly in tropical than in temperate climates, 
and that this explains why the organic matter in the Hooghly water 
is smaller in amount than that of the London waters, both of river 
and wells in their natural state. 
But we have to consider not only the quantity but the quality of the 
organic impurity. We can scarcely expect to go more minutely into 
this than to endeavour to ascertain the relative proportions of vege- 
table and animal matter, and to get some idea of their state or of the 
stage of decomposition in which they exist in the water. The 
chemical constitution of these gives us some aid in this enquiry, the 
main constituents of vegetable compounds being carbon, hydrogen 
and oxygen, those of animal substances containing nitrogen in 
addition ; a statement which, though not strictly exact, is sufficiently 
characteristic, so much so, that by azotized or nitrogenous substances 
are generally understood compounds of animal origin. The ultimate 
products of the decomposition of non-nitrogenous organic matter in 
presence of oxygen, namely water and carbonic acid, of course give us no 
help in this enquiry, nor are the intermediate products likely to be 
