1867.] with the supply of water to Calcutta. 3 



So far as general observation could go, having been engaged in 

 collecting and examining the river water from 1st May to 14th June 

 for the purpose of ascertaining the amount of tidal contamination, 

 I had abundant opportunities of judging of the physical characteristics 

 of the water and observed nothing particular except a comparatively 

 slight, somewhat fetid smell, which contrasted distinctly with the very 

 decidedly worse smell of the water after the rains had come on, and 

 of which the personal use of the river water gave me a vivid illustra- 

 tion. Other differences I have already noticed in the earlier part of 

 the communication, all suggesting the greater proportion of organic 

 matter in the water of the rainy season, at least in the earlier part of 

 it. Moreover looking to the absolute weight of organic matter, I had 

 only found even in the worst of the tanks, when their water was low 

 and putrid, four or five grains in 100,000 fluid grains of Avater, equal 

 to rather less than three or four grains per gallon ; while the river 

 water at any season was much superior to these in smell and colour, 

 even during the rains, that is after the mud had settled. 



Yet as these observations might not be sufficiently precise, experi- 

 ments were instituted to endeavour to determine the question. The 

 oxidation of the organic matter by permanganate of potash offered 

 the readiest and easiest way of examination, and was applied to 

 various samples of water, more particularly to determine the rapidity 

 of change after collection. And it did indicate a rapid change even 

 in course of a day or two, indeed the greatest amount of change took 

 place within the first 24 hours. But it has already been pointed 

 out that this test indicates the proportion only of certain kinds of 

 organic matter, and gives no information as to the total amount. It 

 may even indicate more oxidizable matter after the amount by weight 

 of organic matter has diminished, as was really the case in some of 

 the experiments made. This will be seen in the case of the mixtures 

 in the succeeding table, in which the proportion of oxidizable matter 

 diminished for the first few days, and then increased decidedly, 

 afterwards diminishing again. In No. 3 mixture it increased to a 

 large extent up to time of writing this, and no doubt would diminish 

 afterwards. The great extent of change in this case is accompanied 

 by a great diminution in weight. 



The question at issue, however, was the amount by weight of 



