nature; non-nitrogenised matter. It appears to me, however, that there is a tendency generally to make too much of this distinction between matter of vegetable and of animal origin, it being often spoken of as if the organic matter were of little importance, if it could be shown to be vegetable matter. Now it may be admitted that most probably water tinged by peaty matter consisting of the ordinary humous class of acids and salts, may be not at all even injurious to health, and that water flowing over or percolating through the soil of mountainous districts or others bare of vegetation, where there is little herbage and much earth or rock, may be very pleasant and wholesome. But citing such cases is only evading the question. It does not follow that water draining off cultivated fields or dense jungle, or flowing between banks covered with luxuriant and rank vegetation, will be equally harmless. Putrefying animal matter is very offensive, but putrefying green vegetable matter, though not so disgusting in idea, is scarcely less offensive. Nor, be it remembered, are the poisonous properties very much dependent or connected with a disgusting taste or smell, and the most powerful poisons come from the vegetable kingdom.

In my first communication, the oxidising action of the atmospheric air dissolved in river water was brought forward as a powerful agency for purifying the water. And though Dr. Frankland's results were quoted as strikingly illustrating it in respect to the Thames waters, yet such observations are by no means new. And it must again be observed that the high temperature of the climate will materially assist this action. No doubt it assists putrefaction, fermentation also, and in some cases this may take place in a river, when its course from any cause is rendered very slow. But in the case of the Hooghly the tides cannot fail to act beneficially; twice every day damming back the water and again retreating, enabling the river to flow with increased velocity, increasing the motion amongst its waters and constantly changing the surfaces exposed to the air. This is just one of those agencies that escape general observation it does not exhibit itself to the senses, yet it must be remembered that it is by the oxygen dissolved by water, small though that be in amount, that animal life is preserved in the waters no less than in the ordinary atmosphere.

The organic matter remaining in the mother liquors, after having as well as practicable crystallised out or otherwise removed the mineral saline constituents, is, in the case of the hot season river water, of a pale brownish yellow colour, with a comparatively faint, somewhat urinous smell; that from the tanks and river water of the rains is of a darker brown colour and a more excrementitious smell: a smell, in fact, similar to that of Guana. So far as sensible properties go, the latter is the more disagreeable, and according to the results of experiment mentioned in the first paper, probably at least equally nitrogenised. At present it would be difficult to speak more positively on the subject.

In my first paper, and more particularly in the abstract of it furnished for the proceedings of the Society, I made some remarks in connection with some of the tanks which my own observations will not bear out. This was the result of haste and inadvertence, which will now be corrected. Some of the best tanks, General's Tank more particularly, are probably equal to the river water in purity at some seasons, and superior to it at others. Tank water deteriorates during the hot season from putrefactive fermentation; the river water proper improves from oxidation, but near Calcutta deteriorates from sewerage and tidal water. Tank water improves during the rains by dilution with rain water, and the animal and vegetable life in it preserves the proper balance, removes decaying matters, and prevents putrefaction to any great extent at least, this is the case in good tanks. General's Tank seems a well kept aquarium; it abounds in animal life though its water has often a slightly putrid flavour, this is easily removed by exposure. But even the water of that tank is not, in my opinion, equal in freedom from organic impurity to the river water proper, taken during the dry season at ebb tide.

General Conclusions.

Before closing this communication, it may be well to make a few general remarks as to the conclusions to be drawn from the data obtained. It must have been observed that there is some uncertainty connected with the subject of the organic matter. Persevering enquiry may in time enable us to remove much of this uncertainty, but at present we can only draw conclusions from the most certain grounds we possess. Probably all will agree that it is advisable to get

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 town 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