which receives the greater part of the sewerage of Calcutta requires less oxygen to destroy the products of putrefaction than the best tank waters of Calcutta ; and the water of the salt marsh to the east of the town, called the Salt Water Lake, requires only about as much as that of General's Tank of the same age; for taking the rate of improvement between 3 hours and 26 hours, General's Tank water at 19 hours old would require .1626 grain oxygen. Results like these have led me to set but a small value on this favourite process, and induced me to turn to others promising more trustworthy indications.

The fundamental point with respect to the organic matter is the same as that connected with any other constituent, namely, its proportion by weight, ascertained as accurately as practicable. The method of doing this has already been detailed in the first paper, and I have only to repeat that, with a fine balance, patience, and care, it gives fairly satisfactory results. Attention to details is advisable to procure uniform results. Of the ordinary river or tank waters, I usually evaporate from 10,000 to 40,000 grains, according to the kind of water, contriving so as to have 4 to 6 grains of dry residue, beside 3 grains of dry Carbonate of Soda* added to the water, when put to evaporate. At one time I did not use the soda for some kinds of water, as unnecessary, but now I use it always. It makes the results more accurately comparable. These quantities are sufficiently large for the crucible, which holds conveniently about 200 grains of carbonic acid water, but requires to be twice or thrice filled up. A larger crucible would be more convenient, in which case once might do.

The river water of the cold season of 1865-66 had been kept over from two to four months, and the results as to organic matter therefore were doubtful. These will now be replaced by new determinations, all made without delay. There has been no opportunity yet for making new determinations of the hot season and rainy season waters; but I have already, in the "Supplementary Observations," given reason for believing that the delay of from 9 to 16 days in making these determinations, in the case of the hot season waters, cannot have been productive af any serious error. Additional reasons will be given for this opinion presently. There is greater doubt respecting the July and

* Dr. Parkes in his "Practical Hygiene" recommends 30 grains Carb. of Soda! This is surely a misprint. He also recommends to restore the carbonic acid lost by ignition by adding solution of carbonic acid or carbonate of ammonia, This is a mistake: the results by carbonate of ammonia are totally wrong.

August waters, which stood in most cases about five weeks, to allow the fine clay to settle. This is a special case and will require further remark; but at present, as I wish to present a view of the whole, they will be taken as they are in the construction of the following table. Many more determinations of organic matter were made, particularly of the waters of the rainy season, but these were made for special purposes and with various modifications of the process, so that they were not comparable. Only those are given in the table which were made by the plan already specified, and so far as this is concerned, they are therefore comparable. These variations in plan were chiefly tried in September and October, and so unfortunately no results for these months can be introduced into the table. But as an illustration of these variations, I may instance the case of the water of 6th October, which, cleared by Hydrochloric acid, gave 1.05 grains, and cleared by sol. potash, gave 3.22 grains organic in 100,000 grains of water. The addition of a little of either of these causes the mud to settle, and admits of filtering the water clear in course of a few hours. But as there is much organic matter adhering to or combined with the clay or other earthy matter of the mud, the acid or alkali acts upon this and brings it into solution.

TABLE V.

Shewing the amount of Organic matter by weight in 100,000 grains of the Hooghly water.

Ebb. Flood. Grains. Grains.

With reference to this table, a few remarks may be made. The rainy season waters were taken during Neap tides at ebb. Probably they would not have differed much, though they had been taken at spring tide during flood. Special remarks will be made on these afterwards.

The numbers attached to the waters of November and January, shew that the surface water contains more organic impurity than the deep, and that there is a very decided excess of this during flood tide as compared with ebb.

The May and June waters are the old ones,-open to future emendation as to quantity of organic matter. But even these indicate a still larger excess of organic matter during flood tide. And comparatively small though the amount of organic matter be, compared with those hitherto generally received, they shew the influence of the tides in bringing up organic matter, as has not been sewn before.

I have already observed that the opposition, with which my statements as to the small amounts of organic matter originally met with, has been now in a great measure withdrawn; yet it may be desirable to make a few observations on the subject. Having regard to the delay in examining the waters of the cold and hot seasons, I abandoned those in which the water had stood from two to four months, (the cold season samples); but did not think that the delay of from nine to sixteen days would materially affect the correctness of the results from the hot season waters. Reasons have already been given in the Supplementary Observations for this, to some probably not sufficiently conclusive, so I shall in the first place give the results of the examination of another class of waters, which may have some bearing on the subject. These are the waters of the Salt Water Lake and of the Circular Canal.

The Salt Water Lake is a large salt marsh of about one-third of the degree of saltness of sea water, about two miles to the eastward of the boundary of the town. From Entally, near the Circular Road, a canal proceeds eastwards towards it, called the Baliaghatta Canal. At this extremity it forms a cul de sac, but is joined about half way in its course by another branch which proceeds from the river at the northern extremity of the town, and in its course, enclosing all the northern part of the town, at length joins the Entally branch. These

Canals, I am informed by the town surveyor, Mr. Rowe, receive by far the largest proportion of the sewerage of Calcutta, the course of drainage, except for a narrow space along the river, being towards the east, or from the river towards the lake; so to these sources of supply I went for specimens of impure water. The Canal water flows eastward or westward according to the relative height of tide in the Hooghly or Bidyadurrea with which it communicates. The following table gives the results of the examinations.

The most remarkable thing exhibited by the table is the small quantity of organic matter in these waters. The excess in the case of the 14th February water, was, there can be little doubt, owing to the water having traversed the whole length of the circular canal, passing over its putrid mud and carrying with it the sewerage from the numerous drains which enter it. The small quantity of organic matter in the filthy looking water of the marsh, full of gelatinous

* Probably some unobserved error in this case.

42.60

2.51

2.58

...

32.95

1.78

...

34.90

1.53

looking rank vegetation, is very striking. I got in June last year 20 grains organic in 100,000; but besides that I doubted the correctness of the result, considering it perhaps over-estimated, the difference of season must be taken into account.* Besides this is a strongly saline marsh. The comparatively small quantity in the canal water is also remarkable, and shews how difficult it is to increase greatly the amount in the comparatively pure water of the downward flowing stream.†

And here it may be well to consider the amount of organic matter which the river can receive from the sewerage of Calcutta. First, we have to consider the amount of water carried by the Hooghly, for the data for which I have to acknowledge my obligations to Mr. Leonard. He estimates that at the lowest season, the river, through its tributary affluents, receives only about 2,000 cubic feet per second, but 8 or 10,000 cubic feet more by percolation from its banks, or say from all sources 10,000 cubic feet of water per second, equal to 8€4,000,000 cubic feet per day. Mr. Clark, in his report on the water supply, proposes to distribute 6,000,000 gallons per day or even ultimately 12,000,000 gallons. Now let us take the highest of these quantities; at a rough estimate this is about 2,000,000 cubic feet, and its proportion to the volume of the river water is as 2 to 864 or 3 part. The greatest amount of organic matter I found in the filthiest ditch in Calcutta at its worst in June was about 24 grains per gallon. Now supposing all this large quantity of water was daily poured into the Hooghly in the state of sewerage of this degree of concentration, it would be only part of 24 grains of organic matter to each gallon of river water or about .05 to .06 grain per gallon. Even supposing that the amount of water carried by the river has been over-estimated, and that it amounts to only one-half or one-fourth of the quantity stated above, the proportion of organic matter added by the sewerage would not exceed one-fourth of a grain per gallon at most, during the hot season.

Dr. Parkes states that 12 to 40 or 50 grains per gallon is not uncommon. It would be necessary to know how such results were obtained, before admit. ting them.

Probably if the water of the canal, instead of having been collected at full tide, had been taken when the river water was beginning to flow into the canal, it would have contained more organic matter. An attempt indeed was made to get such water two days before (18th February), but the proper time was not hit.