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appear to have attracted the attention that was due to them. I shall also draw my own conclusions from my results, stating at the same time with what amount of confidence they are made.

Inorganic constituents.

In the original communication, on account of an unforeseen and unexpected source of error which vitiated some of the results and therefore rendered the series incomplete, only a general view of the relative proportions of alkaline and earthly salts at the different seasons, taken from the tables in Dr. Macnamara's Report, was given. It may be of interest to state the nature of the source of error, then only hinted at. It occurred in the case of the waters of December and February, greater part of which had been kept in green glass stoppered bottles till the month of April, which, on analysis, gave results so peculiar as to excite surprise. The same peculiarities were found in some of the analyses of the river water of August, in even a more marked degree. After not a little perplexity and trouble, it was ascertained that this arose from the action of the water on the glass, dissolving the glass in such proportion as altogether to vitiate the result as regards the proper constituents of the water; it having been ascertained that the silica, the alkalies, and the iime of the glass were all added in notable proportion to the constituents of the water. It was the very large proportion of the silica obtained that first drew attention to the subject. Not being specially connected with the object of this paper, it is not necessary to notice it more particularly than to observe that there can be little doubt, but that it is due in great part to the increased activity given to the solvent action of the water by the high temperature of the climate, though indeed it occurred to a sufficiently decided degree even during the coolest months. There is probably little doubt that this circumstance has in many cases introduced error into water analyses unobserved. The analyses, in the present case so vitiated, were rejected and new ones instituted as the season gave opportunity.

For the purpose of comparison, the most complete plan would be, to ascertain the amount of each basic and acid constituent and state these in detail. A very general, or rather the general plan hitherto followed by chemists, has been to allot the acids and bases to cach

other, it may be by some conventional plan or according to some favourite theory, and represent them in the state of neutral salts. And as each chemist may follow his own particular plan, the same analysis may be represented in very different ways. As it is simply impossible to say in what way the acids and bases are united to one another in solution, it is very much better to state them separately; and I was glad to find that Professor Dr. W. A. Miller expressed the same opinion in his paper formerly referred to. But for general purposes a full statement of each constituent is unnecessary, and when numerous samples have to be examined, is very laborious. It is generally sufficient to classify them, or select a few of the most important and characteristic constituents or properties. In the case of the mineral constituents, their total amount, the quantity of chlorine or of sulphuric acid, the proportion of earthy salts, that is, of lime and magnesia to the alkaline salts, are, singly or together, all more or less suitable according to the nature of the water to be examined. The soap test formerly noticed is a very favourite method, from the ease of its execution. I have applied it in some cases, though the nature of my enquiries led me generally to have recourse to other methods.

The following table gives a view of the constitution of the river water at the various seasons, classified in a way that seems to me very suitable for comparing different samples. The principal mineral constituents are the alkalies, potash and soda, and the earthy, lime and magnesia,—soda being the most abundant alkali, and lime the principal earthy constituent. These bases are combined with carbonic acid in much the larger proportion, and in smaller proportion with hydrochloric acid, sulphuric acid and perhaps organic acids. The carbonates of lime and magnesia are kept in solution by excess of carbonic acid, and when the water is boiled or evaporated to dry dryness, by far the greater part, indeed all except a very little of the lime and magnesia, are separated insoluble. These remarks apply to the river water proper; during the hot season, when tidal influence prevails, the constituents of seawater make their appearance; then sulphuric acid is increased a little and magnesia still more; and hydrochloric acid and soda (or chlorine and sodium as common salt) are largely increased in quantity.

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In Table I. the alkalies are exhibited as if they were all in the state of hydrochlorates of potash and soda, or more correctly chlorides of potassium and sodium, chloride of sodium or common salt being the best type of such compounds, and the one most familiar to us, and practically most important. The earths are exhibited as if they were all in the state of carbonates of lime and magnesia, these compounds being also the most familiar ones. By this arrangement, the relative proportion of these constituents at different seasons can be easily compared. I am not aware that this plan has been used before, but it seems to me a good one, particularly when combined with the results given in Table II.

* Silica mixed with more or less clay.

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This table shews the indications of the soap test already noticed in the first paper. The total hardness is the effect produced on soap by all the salts of lime and magnesia present, and all the carbonic acid. and silica; the permanent hardness is that left after boiling, and is produced chiefly by the lime and magnesia not separated in the insoluble state, but still remaining in solution. Another column exhibits the proportion of Chlorine calculated as if it were all in the state of chloride of sodium or common salt. The chlorine is in small quantity except when tidal influence prevails.

So far as regards mineral constituents, the water of the Hooghly at Calcutta varies greatly according to the season. Compared with the waters supplying London, the solid contents during the rainy season are much smaller, and the total hardness much less; and even in January and February, these are somewhat under those of the London waters. As regards permanent hardness, the Hooghly water is very decidedly superior to the London waters probably all the year round, except possibly during the hot season at flood tide, though that latter point is at present somewhat uncertain. But the temporary hardness is easily removable; and for economical use, except during flood tide

*It must be remembered that these results are for 100,000 grains water. For an Imp. gallon of 70,000 grains multiply by 7 and move the decimal point one place to the left.

of the hot season, as regards mineral constituents, the product of the Hooghly may be considered very good water. It will be compared with the Calcutta tank waters afterwards.

Organic constituents.

The attempt made by experiments with artificial mixtures to imitate the composition of the waters of the hot season, and ascertain the probable amount of change in the organic matter by keeping, as narrated in Part II. " Supplementary Observations," was not continued, partly because all the circumstances of the case could not be imitated, and partly because the plan did not seem to be considered satisfactory to those who objected to the correctness of my results in this particular. It appeared to be better to continue the observations, taking care to avoid delay in the process for estimating the organic matter more particularly. Besides, recently the objections to the correctness of my results have been in a great measure withdrawn,* and it is hardly necessary for me to do anything more in the way of directly answering objections, as it was never my object to criticise the labours of others, but simply to state my own, carefully obtained by methods of procedure the most correct and reliable known, up to the present time.

In the original paper I considered the various methods of ascertaining the nature and amount of organic matter in water, and discussed their several merits; and a few further remarks will now be made on the same subjects. The amount of organic matter by weight came first in order, but I shall at present postpone it, until the plan of oxidation by permanganate of potash has been noticed.

This plan has come greatly into favour, chiefly I suppose from its facility of application, a very valuable recommendation, provided its other merits be assured. In the original paper I gave it a qualified and guarded approval; the result of numerous experiments made since has not increased my estimate of its value, nor has that experience, and reflection thereon, led me to concur in the generally favourable estimate in which it is held. It is said "that it is not improbable that "the substances most readily oxidised, are just those most likely to be "injurious in their effects upon those who drink the water." This is Dr. Miller's remark. Others "believe" that the most pernicious are

* Indian Medical Gazette, Calcutta, 1st January, 1867, p. 14.

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