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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 illustration. 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 water, 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, experiments 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

organic matter present. It was impossible of course to get the hot season water in its original condition, but experiments could be made with river and tank water, and with mixtures intended to imitate the real or supposed peculiarities of hot season water. These could be examined to ascertain the amount of change produced on them by keeping. Accordingly experiments were made the results of which are exhibited in the following table.

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The Mixtures were composed of river water of the hot season three or four months old and of recent river water with a little Salt Lake water, No. 3 containing also Tank water; with these were mixed the specified proportions of sewage water which had been collected on 8th September, and, as tried on the 9th, contained 27.33 grains organic matter in 100,000 fl. grains.

It will be observed that in the organic matters oxidised by the permanganate of potash there is a distinct diminution early, even by the lapse of a single day, as indicated by the smaller quantity of oxygen

* Introduced after date of paper.

Evidently an error of Expt. The organic matter could not increase.

Exp. faulty. Enough of Carb. Soda had not been used. Result could not have been less, but probably would have been greater, had it been correct.

required subsequently; afterwards the diminution is slower, or in some cases even an increased quantity of oxygen may be required, from changes taking place in the water causing the production of a larger quantity of readily oxidisable matter. This therefore gives no indication of the weight or actual quantity of organic matter present. The weight of organic matter ascertained by experiment however, indicates in some cases a rather rapid diminution at first. But this is only to a small amount, except in the case of highly decomposable or putrefying liquids, such as sewage or mixtures containing much sewage. Calculation will show that the loss of weight of organic matter in mixtures Nos. 1 and 2 is less than would have been sustained by the constituent proportion of sewage water in them. In No. 3 probably the vegetable matter of the Tank water added caused the more rapid and extensive decomposition.

The loss of weight in the mixture No. 3 is 3.4 grains in 8 days, being fully more than half of the original amount; in Nos. 1 and 2 it is only only 1.8 grains and 0.3 grain respectively.

But the river water at no time could contain anything like the proportion of sewage that these mixtures did, such as one-fifth, oneeighth or even one-twelfth of sewage, the smell alone of such mixtures, makes the supposition quite inadmissable; besides a comparison of the size of the river with the amount of drainage of the town would show that such a proportion was quite impossible. The amount of liquid discharged by the drains compared with the volume of water in so large a river must be insignificant.

the results of others.

But instead of citing results of my own, which if incorrect may be supposed to be all equally incorrect, it may carry more weight to quote The older determinations of organic matter are generally of no value whatever, and I shall refer only to the most recent and trustworthy. I have already quoted Dr. Frankland's results with the London waters, but as all these are of water filtered for distribution they may be considered not quite comparable. Another example I shall adduce from the paper of Lawes and Gilbert in the Journal of the Chemical Society already quoted. They give tables of the composition of the Rugby sewage from May 1862 to October 1863, shewing that it contains in solution from 7.6 to 8.35 grains per gallon, and also a statement of the amount in the River

Wandle before receiving the Croydon sewage which is 1.44 grains per gallon, and after receiving it which is 2.08 grains per gallon. According to this the estimates of organic matter to the extent of 8 or 10 grains per gallon in the Hooghly water during May and June shew that it contains fully more than the liquid part of the Rugby sewage, and this in a tropical country.

I do not wish it to be understood that I maintain the perfect accuracy of my own results. The oversight in not examining the samples speedily enough after collection must be admitted, though from all that I have been able to learn from the experiments instituted for the purpose, the error cannot be a great one. There was no great delay in examining the water of the hot season,-about ten or twelve days, and this caused by the time and attention taken up in examining the influence of the tides in numerous samples. There was greater delay with the water of the rainy season, probably about a month with the first samples in July, about a week or 10 days with those of August. This was caused by waiting for the settlement of the very finely divided clay, the presence of which was very unfavourable to the accurate estimation of the organic matter. Recently I have found that the addition of a small quantity of hydrochloric acid causes the mud to settle so rapidly that the water may be filtered clear in course of a few hours: solution of potash or soda and milk of lime do the same, but the water cleared by these reagents seems to contain a different proportion of organic matter than that cleared by simple subsidence. It is of less importance, as the question at present is not respecting the water of the rainy season. The samples of December and February water circumstances prevented me from proceeding with, and they were preserved in stoppered bottles and probably not much done with them till April. The results are consequently more doubtful, though I do not suppose that they are very wide of the truth. As the season advances, should circumstances admit of it, I shall not fail to repeat the analyses, in order to get unobjectionable results.

The observations made during the last month enable me to add a little to my former statements respecting the effect of the change of seasons on the river water. The increase of organic matter from the rains seems to be chiefly of the more soluble and putrescible kinds; as

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