JOURNAL OF THE ASIATIC SOCIETY. PART II-PHYSICAL SCIENCE. No. I.-1867. EXPERIMENTAL INVESTIGATIONS connected with the supply of WATER from the Hooghly to CALCUTTA, Part II, being Supplementary Observations; by DAVID WALDIE, Esq. F. C. S. &c. [Received 28th September, 1866.] In the preceding remarks I have directed attention to the discrepancies between my own results as to the quantity of organic matter by weight in the Hooghly water and those given in Dr. Macnamara's Report, and I have also made some pointed observations on the very doubtful accuracy and unsatisfactory nature of the results generally given by chemists respecting organic matter in waters, except some of the most recent. For though I have found that the process detailed in the previous part of my paper is older than I then supposed, having been recommended by Mr. Dugald Campbell in 1856 as suggested by Dr. Clark, and that an analogous plan was given by Abel and Bloxam in 1854, though imperfect, yet these plans seem either to have been little known, or neglected, or imperfectly carried out. Some analysts. indeed of later date do not even attempt to estimate the amount of organic matter at all, apparently despairing of reliable results. But the process given, I believe, yields the most trustworthy results hitherto obtainable, if properly performed. * *Journ, Chem. Soc. Vol. IX. 1856, p. 51. But if the estimation of the organic matter in waters is to be of any value at all as a means of judging of their salubrity, it is essential that it should be done accurately. If it is to be a fundamental datum on which Municipalities are to choose or reject certain waters for the supply of large towns, that sanitary boards are to draw conclusions from as to the healthiness of certain localities for the residence of troops or other collections of human beings, and on which medical men and hygeists are to reason respecting the origin of disease or the maintenance of health, it is unnecessary to say that it ought to be ascertained in a reliable manner. In the case of my own results, differing so widely from those referred to, the question occurs, is there no way of accounting for them or reconciling them? One cause has been suggested to me independent of correctness of method of analysis or of accuracy in its execution, namely the age of the water when examined, that is the length of time which had elapsed since the water was taken from its source. High chemical authority has been adduced for the necessity of setting about the analysis with the least possible delay, on account of the chemical changes which the water would undergo by keeping, which would result in a diminution of the quantity of organic matter present. The validity of the caution I am not disposed to deny, neither am I prepared to deny that in my own operations this point was not always sufficiently attended to. Indeed it had not particularly attracted my attention; except as regards gaseous constituents the point had not been particularly noticed either in text-books or monographs I had seen, and the consideration that the organic matter collected by rivers had already been freely exposed to decomposing agencies, so that probably what remained was not readily decomposible, confirmed as this was by my own observations while operating, led me not to attach much importance to it. Still it appeared that there might be a change of considerable amount shortly after collection which had passed unnoticed, while afterwards the water remained less liable to change. A small change, experiment shewed, did occur speedily, but the present question did not refer to a small change but to a large one, and it was desirable if possible to ascertain to what amount it might extend. The question principally concerned the waters of the hot season and of the rainy season. 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. 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. |