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drinking water containing as small a quantity of organic matter as possible, and more particularly as small a proportion as possible of that which is of recent origin. If, this being kept in mind, we take up the question which seems to have been considered of greatest practical importance by the Calcutta community, namely, can the supply be safely taken from the river at Cossipore? we can scarcely answer it in the affirmative. My results, as has been pointed out, show that there is a very distinct increase in the quantity of organic matter in flood over ebb tide, even during the cold, but still more during the hot season. How far this may be due to the proximity of Calcutta, could only be ascertained from extended observations; but as the town must supply a considerable quantity of putrefying and putrefiable matter and that of recent origin, in the absence of evidence indicating the contrary, it would be desirable to avoid taking it from that locality. What is the smallest distance up the river at which this source of contamination is not appreciable, is a point that could be determined only by observations during the hot season in various circumstances and places. But it is evident enough that the further up we go, the more certain are we to avoid this source of contamination.

But though this is an important question, it is not the only one: it seems to have occupied almost exclusively the attention of that portion of the community who have taken an interest in the subject, while another, and in my opinion an equally important one, has scarcely if at all been noticed; and that is, what is to be done with the muddy water of the rainy season? If we look to the amount of putrefying matter as indicated by the permanganate test, or even as observed by the senses, the water, for the first two months at least of the rains, is worse than the flood tide water of the hot season; if we look to the two as we have them, each with its suspended mud, the rainy season water is greatly the worst. If we consider the quantity of organic matter actually dissolved in the water, probably the hot season water contains most, though this at present is not quite certain, and it is also somewhat doubtful if it be so bad in quality as in the rainy season water. Of all points in the enquiry, this is the one involving the greatest doubt and difficulty, and I should feel it quite impossible to give a decided opinion on it, without again examining the water during that season.

And what

makes this point of chief importance is this, that though the towu contamination may be avoided by going up the river, this cannot.

That flood waters (that is, floods produced by rain fall) are most impure, as regards organic matter, is now a recognised fact in England. It is by no means a new observation. In the Report on the Metropolitan Water Supply by Messrs. Graham, Miller and Hofmann, presented to the Secretary for the Home Department in June 1851, this point is repeatedly noticed and the remedy for it discussed, though from the nature of their remarks it is evident that the amount of finely suspended mud, and the degree of its putridity, have probably been much smaller than those of the Hooghly water in the rains. It must be remembered that while in England there are numerous small floods, here we have but one large flood in the year, washing down the accumulated refuse of seven or eight months. It is true that the large quantity of rain dilutes the muddy mixture, and, so far as matter in actual solution is concerned, improves the water. Still we have it loaded with mud, part of that in a very fine state of suspension, very slow in settling, and which cannot be separated by any ordinary filtration. And as the finely suspended clay contains organic matter, putrid or putrefiable, the water must be deprived of it to be rendered fit for use.

The subject has engaged the attention of the Engineer to the Justices, and in his Report on the works for the supply of water to Calcutta, he details the plan for meeting the difficulty alluded to. He admits the difficulty, for the says, para. 28, "The muddy character of the water to be dealt with is an unusual feature in works of this description and necessitates peculiar aud special arrangements being provided." Let us see what these are.

It is to be settled in large tanks 6 or 7 feet deep for 36 hours; then the upper portion, to the depth of 4 feet, is to be drawn carefully off to the filter, after passing through which it is conveyed to covered reservoirs for storage, whence it is to be distributed as required. The filter is composed of sand and gravel, and also, according to Mr. Clark's original proposal, a layer of "Spencer's magnetic carbide." The object of this is to purify the water from organic matter, and it is also said that it removes the suspended matter.

But so far as I can gather from the Report, Mr. Clark seems to

consider that the settling of the muddy water of the rains for 36 hours will put the water on an equality with that of the rest of the year as to the rapidity with which it will pass through the filter, and, I suppose, with or without the magnetic carbide, will supply it in an unobjectionable state. At least I cannot find in the Report any provision made in addition to this for the special case of the muddy waters of the rains, or a single arrangement made to provide against any difficulty in this case.

My own observations on the waters of the rainy season are not at all in favour of the success of this scheme. On the contrary, I have experienced the greatest difficulty in getting the water freed from the finely suspended matter by either subsidence or filtration. After standing to settle for several weeks, it still contained much of this finely suspended clay, from which it could not be freed by filtration in the ordinary way. It seems therefore to be impossible to avoid the conclusion that through the ordinary sand filter the water will pass little changed; or if by any modification it be made effectual, it will pass with such extreme slowness as altogether to interrupt the ordinary supply of the water. And if it pass in its muddy state into deep covered reservoirs provided for it, daily it will deposit a portion of its mud, which will be daily more or less stirred up by the new flow of water into the reservoir, a state of matters which appears to be very well adapted to maintain the water in the state in which it entered, or even to tend to make it worse. Whether the water of this season then will be in a fit state for storage, after thirty-six hours settling and the short time longer necessary for its passing though the filter and being conveyed to the reservoirs, is a question deserving of serious consideration. My own observations lead me greatly to doubt it. It would be rather a serious error, if these fears should turn out to be well founded; for not only would the water be offensive during the rainy season, but, unless the reservoirs were cleaned out, would continue to be so. And in the plan there appears no arrangement for cleaning them, and no facilities for doing so.

There are other and effectual plans for speedily separating the suspended mud from the water, and rendering it easy to be filtered perfectly transparent. These are by chemical precipitants, to some of which I have previously alluded; one well known is alum, in daily

common use.

These have been observed sometimes to increase the quantity of organic matter in solution, but this is probably from imperfect knowledge of the proper way to apply them, rather than essential to their operation.

This enquiry into the water of the Hooghly was commenced, as formerly stated, without any reference to the water supply of Calcutta, but simply as an investigation interesting in a scientific point of view. For even in the single point of the organic matter there is room for the expenditure of much labour and research. It is difficult and not very promising, but persevering enquiry often brings much of interest out of unpromising subjects. The high temperature of the country increases the energy of chemical action, and the comparative regularity of the seasons favours the simplicity of its operation; and thus a country like India affords a field well adapted for the study of the influence of chemical changes on the phenomena of nature. Many of these changes may not be very obvious to those unaccustomed to study them, but they are not the less real on that account, and not the less powerful in their operation on the world around us.

16th September 1867.

Since writing the above, the enquiry has been continued, with the view of settling one or two points left in some degree of uncertainty. These were the amount of organic matter in the river water during the hot season and during the rains. The first of these was the point on which the greatest doubt was felt by many as to the correctness of the results given in my first paper, though little shared in by myself the second was the point on which my own opinion was most undecided. The experimental results will be given in the tables, and comments and explanations will follow.

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The experiments on the hot season waters were made to decide the question involved in the objection raised to my first results, namely, that the organic matter had been decomposed and lost by the delay (of from ten to fourteen days) in proceeding to the evaporation. In the present cases the water was evaporated on the next day after collection. The result shews only from .90 to 1.70 grains organic matter in 100,000 grains of water taken at flood tide of the highest spring tides of the season; less indeed than I had obtained in 1×66. Many other experiments have shown me that there is very little change in the weight of organic matter sustained by keeping it even for several weeks, and least of all in the case of water containing so much saline matter as the river does at this season. The only thing that undergoes rapid change is the deoxidising power of the water as shewn by the permanganate test, but this does not appreciably affect the weight of the organic matter.

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