Quality of the Waters supplied to the Metropolis during the Months of February and April, 1867.

This table is to be read in the following manner :-100,000 lbs. of the Chelsea Company's water contained in the month of February last 28 58 lbs. of solid impurity: the organic matter constituting a portion of this impurity contained 0.433 lb. of carbon. This solid impurity also contained 0.337 lb. of nitrogen in the form of nitrates and nitrites, besides 0.004 lb. of ammonia. The above quantity of water as supplied by the Chelsea Company had been, after its descent to the earth as rain, contaminated with sewage or the manure of cultivated land equivalent to 2,420 lbs. of average London sewage. By gradual oxidation, partly in the pores in the soil, partly in the Thames and its tributaries, and partly in the reservoirs, filters, and conduits of the Chelsea Water Company, this sewage contamination had been entirely converted into comparatively innocuous inorganic compounds before its delivery to

consumers.

A glance at the table shows how vastly superior is the quality of the water of Loch Katrine as compared with that of the best at present supplied to London. 100,000 lbs. of this water contain but 3.28 lbs. of solid impurity, of which only 0.031 lb. is nitrogen in the form of nitrates and nitrites, and 0.002 lb. of ammonia. Further, Loch Katrine water exhibits no sewage contamination, either previous or present.

The nitrogenous organic matter which has escaped the process of oxidation above described, and which therefore still exists in the water at the time the analysis is made, constitutes what may be appropriately termed the present sewage contamination of the water.

The amount of this contamination may be expressed by the number of parts of average filtered London sewage (of the strength above described), which, if contained in 100,000 parts of pure water, would contaminate the latter with the same amount of combined nitrogen. No contamination of this nature has yet been detected in the waters supplied to the metropolis, but the investigations for its discovery have only been made since February last. It will doubtless be consolatory to the consumers of Thames water to know that, although, according to Mr. Bateman, the population within the basin of the Thames above the points at which the water is withdrawn for the supply of London exceeds 1,000,000 persons, the drainage of some 600,000 of whom is poured into the river, the whole of this fæcal matter is so completely oxidized before it reaches the water-cisterns of London as to defy the detection of any trace in its noxious or unoxidized condition. If the average flow of Thames water just above the point at which the London Companies withdraw their supply be taken at 800,000,000 of gallons daily, the drainage of 600,000 people ought to produce a sewage contamination of 2,250 parts in 100,000. It could scarcely be expected that this calculated number should approximate very closely to that obtained by the actual analysis of Thames water, since the calculated number depends upon many contingencies, as for instance, upon the volume of water actually flowing past the points of withdrawal at the time the companies abstracted the water analyzed; and secondly, upon the greater or less retention of the fæcal matters, in the sewers of the towns draining into the river. It is interesting, however, to find that the sewage contamination of Thames water, as determined by analysis, does not differ much from that calculated according to the above data. The analytical table given above shows that the average previous sewage contamination of the water delivered by the five companies drawing their supply from the Thames during the months of February and April, 1867, was 2,355 parts in 100,000 of water, the amount calculated from the number of persons draining into the river being, as just mentioned, 2,250 parts in 100,000 of water. As summer advances and aquatic vegetation becomes vigorous in the bed of the Thames and its tributaries, this coincidence of calculated and analytical results will probably be disturbed, as the water-plants can scarcely fail to withdraw an appreciable amount of nitrates and nitrites from the water, thus diminishing the quantity of combined nitrogen and consequently of previous sewage contamination as determined by analysis.

The second important class of impurities contained in water used for domestic purposes consists of certain mineral salts which possess the power of decomposing soap. These substances are the hardening or soap-destroying constituents of water. From a purely sanitary point of view they are of less direct importance than the

organic impurities; still, by rendering efficient ablution and thorough cleanliness difficult of obtainment, they doubtless indirectly affect the health of communities supplied with waters in which they are present in considerable quantities.

The chief hardening ingredients in potable waters are the salts of lime and magnesia. These salts decompose soap; forming curdy and insoluble compounds containing the fatty acids of the soap and the lime and magnesia of the salts. So long as this decomposition goes on the soap fails to produce a frothiness in the water, but when all the lime and magnesia salts have been decomposed by the action of the soap, the slightest further addition of the latter produces a lather when the water is agitated; but this lather is again destroyed by the addition of a further quantity of the hard water. Thus, the addition of hard water to a solution of soap-or the reverse of this operation-causes the production of the insoluble curdy matter above mentioned. Bearing this in mind, it is easy to understand the process of washing the skin with soap and hard water, which may be thus described:-First, the skin is wetted with the water, then soap is applied; the latter soon decomposes all the hardening salts contained in the small quantity of water with which the skin is covered, and there is then formed a strong solution of soap, which penetrates into the pores of the skin. This is the process which goes on whilst a lather is being produced in washing, but now the lather requires to be removed from the skin; how can this be done? Obviously only in one of two ways, viz. by wiping it off with a towel or by rinsing it away with water. In the former case, the pores of the skin are left filled with soap solution; in the latter, they become plugged up with the greasy curdy matter which results from the action of the hard water upon the soap solution occupying the pores of the cuticle. As the latter process of removing the lather is the one universally adopted, the operation of washing with soap and hard water is perfectly analogous to that used by the dyer or calico printer when he wishes to fix a pigment in the pores of any tissue. He first introduces into the tubes of the fibre of calico, for instance, a liquid containing one of the ingredients necessary for the formation of the insoluble pigment, this is followed by another liquid containing the remaining necessary ingredients, the insoluble pigment is then produced within the very tubes of the cotton fibre, and is thus imprisoned in such a manner as to defy removal by subsequent washing. The process of ablution, therefore, in hard water is essentially one of dyeing the skin with the white insoluble greasy and curdy salts of the fatty acids contained in soap. The pores of the skin are thus blocked up, and it is only because the insoluble pigment produced is white that such a repulsive operation is tolerated. To those, however, who have been accustomed to wash in soft water, the

abnormal condition of the skin thus induced is for a long time extremely unpleasant.

Nevertheless, opinion is not quite unanimous as to the advantages of soft water over hard. Some persons consider hard water to be necessary for the supply of the calcareous matter of the bones, others believe soft water to be peculiarly liable to attack and dissolve the lead of the pipes through which it is conveyed, or of the cisterns in which it is stored.

An examination of the grounds upon which these opinions are based would completely refute them, but the limits of this article do not permit of such a digression, and I must therefore content myself with a mere allusion to one or two facts in connection with them. First, as to the necessity of hard water for the supply of the calcareous matter of bones. If it be assumed that a man drinks daily half a gallon of Thames water, he obtains from it 3 grains of lime chiefly in the form of chalk. This amounts to not quite 3 oz. per annum, which does not seem to be a very large contribution to bony matter. Now suppose the use of this water to be discontinued and that no part of it is replaced by bitter beer, which always contains far more lime in a given volume than Thames water; but we will assume that the individual consumes one-third of a pint of milk per day, he then receives in this quantity of milk more lime than his system can acquire from two quarts of Thames water. Then, as to soft water attacking and dissolving lead; it is by no means true, as a general proposition, that soft water does attack and dissolve this metal. The very soft water of Loch Ness, as supplied to Inverness, does not attack lead, as evidenced by the unimpaired condition of lead pipes through which that water flowed for six years: neither does the soft water of Ennerdale Lake, supplied to Whitehaven, attack lead. Even those soft waters which do attack the metal, such as those now supplied to Glasgow and Manchester, only do so when the surface of the lead is clean and bright. The action soon ceases, in fact as soon as the metal becomes tarnished the pipes are protected, and no complaints of any symptoms of lead poisoning have for the past ten years been heard from these large cities. Lastly, a sample of very soft water taken from one of the principal streams from which it is proposed to supply London has no action even upon clean and bright lead. Notwithstanding the numerous researches made in connection with this subject, the causes of the attack of lead by water have not yet been completely elucidated; it has, however, been established that the presence of oxygen and the comparative absence of carbonic acid in the dissolved gases are essential conditions to this action. Messrs. Graham, Miller, and Hofmann, in their report on the Metropolitan Waters in 1851, first showed that carbonic acid when dissolved in water was a complete protection against lead contamination, and from a series of experiments

recently made I find that 2 volumes of carbonic acid dissolved in 100 volumes of water completely protect even distilled water from such contamination. Rain water as it descends to the earth dissolves atmospheric gases, and this solution is afterwards continued in brooks and rivers. Of the chief atmospheric gases, carbonic acid is by far the most soluble, 100 volumes of pure water can dissolve 100 volumes of this gas; oxygen, on the other hand, only dissolves to the extent of 3 volumes in 100 volumes of water. Nevertheless, owing to the much larger proportion of oxygen than of carbonic acid in atmospheric air (500: 1), water takes up oxygen more rapidly than carbonic acid, and hence freshly fallen rain-water acts upon lead; but when the water flows a great distance through an open conduit, the carbonic acid absorbed finally reaches the protecting proportion, and the action upon lead ceases, although the water retains its original softness. Hence there is no necessary connection between soft water and lead corrosion. Even distilled water left in contact with the air for some time loses its property of acting upon lead.

The hardness of a water is expressed in parts of carbonate of lime, or of its equivalent of other hardening salts, contained in 100,000 parts of the water, and each part of carbonate of lime contained in this quantity of water is generally termed a degree of hardness. This quality of the water may also be more popularly though less accurately expressed by the number of parts of soap destroyed or wasted by 100,000 parts of the water when used for washing purposes. Each degree of hardness indicates the destruction of 12 parts of the best hard soap by 100,000 parts of water. The following table shows the hardness of the London and Loch Katrine waters, according to both these methods of expression: