WATER.— Analysis. Sewage Contamination.

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Ten c.c. of the water to be examined are measuredinto a thin flask of about 150 c.e. capacity, thenabout 30 c.c. of strong sulphuric acid (spec. grav. atleast 1-8) rapidly added, the mixture shaken, andstandard indigo solution run in from a burette untila greenish tint is obtained. This must be done asrapidly as possible, and without any attempt atminute accuracy, the object being merely to ascer-tain the approximate quantity of indigo required.If this be much more than 10 c.c. it is best in subse-quent experiments to employ a proportionatelysmaller quantity of the water, and dilute it to 10c.c. with distilled water. In such experiments addto the measured quantity of water (thus diluted, ifnecessary) the quantity of indigo solution judgedfrom the first experiment to be required, bearing inmind that the first result is probably too low.Having mixed these, measure out a quantity ofsulphuric acid equal to twice their united volume,add rapidly, and shake. If after about a minute thecolour is dark red, somewhat like sherry, moreindigo should be added. With excess of indigo thecolour becomes brown, and then greenish. Thefirst tint of brown should be taken as indicatingthe proper quantity of indigo. Successive trialsshould be made until this tint is obtained on firstmixing, without supplementing the quantity of indigotaken. The quantity of sulphuric acid mentionedabove, viz., two volumes, is that recommended bySutton and used by the writer; but Waringtonsuggests that half that proportion should be used,and says that the tint is more easily observed, being,while the nitrates are in excess, deep reddish,changing when the indigo is in excess to dull brown,and then greenish, the first appearance of brownbeing taken as the critical point. The sulphuricacid must be free from sulphurous acid, and fromnitrous bodies. The former, if present, may beexpelled by heating in an open flask for somehours nearly to the boiling point; and the latterby the addition of a little oxalate of ammonia,and subsequent heating as before.

When organic matter of animal origin is exposedto the action of oxygen, its nitrogen is rapidly con-verted for the most part into nitric and nitrous acids,which, uniting with bases in the soil, form nitrates andnitrites. When the aeration is ample, as in passingthrough a porous soil, nitrates are almost exclusivelyformed. Organic matter of vegetable origin yields nonitrates or nitrites, or mere traces of them. Hence,the proportion of nitrogen as nitrates and nitrites,themselves innocuous, is a trustworthy indication ofthe extent of the pollution by animal matters to whichthe water has been subjected. In upland surfacewater the proportion varies from nil to about 01)5,with an average of about OOl part per 100,000. Insurface waters from cultivated districts the pollutionfrom manure applied in agricultural operations, fromthe drainage of farm-yards, &c., raises the proportionconsiderably—varying from nil, which is rare, toabout 1 part, with an average of about 0-25 part per100,000. In the neighbourhood of shallow wells thepollution, for reasons which have been previouslyVOL. II.

pointed out, is generally more concentrated, and thequantity of nitrates and nitrites is consequentlymuch greater; all proportions having been observedup to 25 parts, from 2 to 5 parts being, perhaps,most frequent. Deep well and spring water con-tains from nil up to about 3 parts, with an averageof 0'5 part per 100,000, that in spring water beingsomewhat lower than in deep well water. In somecases, especially in deep well waters, nitrates aresubsequently reduced by organic matter, or evenwholly obliterated. They are likewise rapidly de-stroyed during the putrefaction of sewage, and henceit is rarely the case that any nitrates are found insewage, unless it is extremely weak, or otherwiseabnormal.

Total Combined Nitrogen. — This number is obtainedby adding together the amounts of the organicnitrogen, and nitrogen as ammonia and as nitratesand nitrites. The first of these represents the actualor present contamination of the water as regards animalmatter, the others the past pollution, and their sumthe total pollution. It must be remembered, how-ever, that all these are minima; we can only saythat the amount of pollution was at least so much,while the evidence of further contamination mayhave disappeared, a loss of nitrogen taking placeboth in the oxidation of organic matter and, asobserved in the preceding paragraph, in the reduc-tion of nitrates and nitrites.

Previous Sewage, or Animal Contamination. —It hasbeen found convenient to express the amount ofcontamination to which a water has been subjected,as indicated by the ammonia and nitrates and nitriteswhich it contains, in some concrete form. For thispurpose London sewage has been selected, and thewater is said to exhibit, in 100,000 parts, a quan-tity of previous sewage or animal contaminationequal to so many parts of London sewage. That isto say, 100,000 parts of the water contain the mineralresidue of an amount of animal organic matter equalto that found in so many parts of average London sewage. For this purpose, which it will be seen isto a great extent conventional, it is assumed thatsuch sewage contains 10 parts of total combinednitrogen in 100,000. This is larger than the actualquantity, a round number being taken for conve-nience ; the amount in 1857, according to Hofmannand Witt, being 8'363 parts, and in 1868 and 1869,according to the Rivers Commission, 7-06 parts.Since rain water contains a certain quantity of am-monia and of nitrates and nitrites, a correction isapplied for this. When the estimation was first pro-posed, this correction was made by the deduction of0032 part, and subsequent analyses of water collectedwith especial care at Rothamsted during the years1869 and 1870 confirm this; the number obtainedbeing '034 part. The previous sewage or animalcontamination of a water is then calculated, bymultiplying the sum of the nitrogen as ammonia,and as nitrates and nitrites, by 10,000 and deducting320. This previous contamination does not indicateactual, but possible danger, arising from the risklest some portion of noxious matter may escape123