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.

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 1866. 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.

One thing is to be noticed in connection with these, namely, that the amount of saline contents is very decidedly smaller than it was in the corresponding season of 1866. This may possibly have arisen from the correct time of full flood not having been caught, but if so, this must have happened on both occasions: and moreover the quantity of saline matter at Ebb tide is also much smaller than in 1866. It appears to me more probable that the river water has contained less saline matter this year. It would be difficult to give an opinion as to the cause of this: besides, the observations were not sufficiently numerous to draw conclusions from with certainty.

The results of the examination of the river water of the rainy season confirm those I have already given in Table III. of the first paper and in Table V. of this. The absolute amount of organic matter is somewhat less than in the waters of the hot season, but, in proportion to the mineral constituents, very much greater. It is to be observed that the waters which were simply settled, or even sandfiltered, still contained clay: the water, clarified by a little sesquichloride of iron, does not give the correct amount of saline matter, therefore in the table this is omitted in these cases, but it was only by such means that the water could be got clear and free from clay, at the beginning of the rains, without delay. The two samples thus clarified were evaporated next day after collection, the others after greater delay.

The waters were also examined for ammonia by the process given in the first paper: the results are exhibited in the following table.

TABLE XII.

Shewing quantity of ammonia in 100,000 fl. grains water.

These results exhibit great variations in the proportion of ammonia, and recent investigations throw some doubt on the trustworthiness of the method employed; besides, it is confessedly an imperfect indication of the amount of nitrogenous matter: nevertheless, so far as they go, they are confirmatory of those already given in table IV. of Part I. The waters of the rainy season yield more ammonia than do those of the hot season, and the tank waters, even the best of them, yield more than the river water. And the previous table also shews that General's Tank,-probably the best of the tanks, or at least one of the best, contains more organic matter than the river water; and the results given connected with Dalhousie Square Tank confirm the opinion expressed before, that the river water rather deteriorates by storage in tanks.

I do not mean to say that these results, as to the quantity of organic matter and the proportion of nitrogenous matter, prove with certainty that the river water is at least equal, if not superior, to even the best of the tank waters; but they are the best means of judging which chemical analysis affords of the quality of water for drinking purposes, and, in the absence of equally good evidence to the contrary, the results lead to such conclusions.

Only in one point have I to say something in modification of the results of previous examination, and that is connected with the water. of the rainy season. In the previous pages I have spoken strongly of the putrid flavour of the river water of the rains, particularly of the early part of the season. This respected the year 1866. In the present year, however, I have not found this putrid flavour, or at least

only to a comparatively very slight degree. At first I was inclined to attribute this difference to some local or accidental cause which had led me into a mistake as to the general character, but after further observation and consideration, I do not think that this is the explanation, but that the water is really different in this respect, this season, from what it was last. For not only has the putrid smell been absent, but the mud has been easier to separate from the water; the particles are not so fine, or at least not so glutinous, and it has not been so difficult to filter clear as it was last year at the corresponding periods; and the quantity of organic vegetable matter produced on standing in bottles has been decidedly less than it was last year. Indeed the river water of the rainy season of 1865 gave much more vegetable growth than that of either of the two succeeding years. Moreover the water of the river did not become muddy nearly so speedily after the setting in of the rains as it did last year: this was noticed particularly, as the muddy water was watched for. Neither after it had become muddy, did it exercise nearly so strong a deoxidating power on permanganate of potash as it did last year: only about one-third of the power; and this is a good indication of putridity. These facts, to which may be added the greater purity of the water of the hot season of this year, seem to indicate some general Both 1865 and 1866 were years cause producing the variation. of deficient rainfall, 62.40 and 60.32 inches; while the year preceding, 1865, was one of large rainfall, there having been 84.22 inches. in 1864. This perhaps may have some connection with the points noticed. The peculiarity is worth attention in the future.

The subject of water analysis, in connection with hygiene, has lately occupied much attention in London. Dr. Frankland has expressed opinions respecting the small value of the oxidation test by permanganate, in accordance with my own and on additional grounds, and also proposed a fuller examination of the nitrogenous constituents as the most important guide. Miles Messrs. Wanklin, Chapman and Miles Smith have also proposed some new methods for determining the nitrogenous constituents. The details of these plans, however, have not yet been published, or at least have not yet come under my notice. The subject is also under experiment by myself, and will have further attention.