222 Mr. Waldie's investigations connected [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 



