supply of excellent water is not confined to its supply from any single variety of source; and that for the purpose of water supply, there are alike good wells and bad wells, good lakes and bad lakes, good rivers and bad rivers. Just, moreover, as the best of river-waters or lake-waters will fall short in regard to an excellency characteristic of well-water, so will the best of well-waters fall short in regard to an excellency characteristic of lake-water or riverwater. The comparison of the one sort with the other sorts of water must be made as a whole. To select arbitrarily the special good point of one sort of water as a standard, to the neglect of the countervailing good points of other sorts, and to measure the quality of the other sorts solely in reference to this selected standard, is clearly the conduct rather of an advocate than of a judge.

2. PURITY OF WATER.

Water, as conceived of by the chemist, is a definite conpound of 100 parts by weight of oxygen, united with 125 parts by weight of hydrogen. So exceedingly difficult is it of production, even if it ever has been produced in an absolutely pure state, that it may be regarded rather as an ideal than a real chemical substance. All natural water, besides the matter, never entirely absent, which it holds in suspension, is a solution of various mineral matters, of various organic matters, and of various gases in the ideal water, or protoxide of hydrogen, of the chemist. Of the different kinds of foreign substance habitually or exceptionally present in natural water, some kinds are beyond question prejudicial, and in particular cases highly prejudicial; most kinds are simply innocuous; while not a few kinds, as saline matter in moderate proportion, and more particularly dissolved aerial matter, are positively beneficial. Looked at, however, from a strictly chemical point of view, any matter whatever foreign to the ideal chemical compound, water, constitutes an impurity of the natural water in which it occurs, whether this foreign matter be a prejudicial or a beneficial constituent, whether it consist of sewage matter,

by which the water is fouled, or of healthful oxygen, by which it is aerated. The chemist uses the word pure, not in the sense of opposite to nasty, but much in the way it is often used in ordinary language, to express the exclusion from one thing of anything else, whether better or worse. Thus, we talk no less familiarly of pure rubbish or pure dross than of pure gold. We speak of pure nonsense as readily as we speak of pure truth, irrespective of the circumstances. that the nonsense would be benefited, though to the prejudice of its purity, by its contamination with a few grains of sense. So chemists speak of pure water, irrespective of the circumstance that, for all the needs of life, the water is benefited, though to the prejudice of its chemical purity, by the presence of its dissolved gases, and of a proportion of dissolved saline matter. Chemists, then, are in the habit of using the word "purity" to signify oneness of chemical nature. Accordingly, in the eyes of a chemist, the matter, for example, of an old copper penny would have its degree of purity equally lowered by the same addition made to it of any kind of other matter, whether of base metal, like tin, or of noble metal, like silver, or even gold. Similarly, in the case of natural water, the small proportions of other substance existing together with the main substance, protoxide of hydrogen, interfere with the oneness of its chemical nature; or, in other words, constitute the natural water a mixture of substances, instead of being a single substance, which is alone looked upon by the chemist as a pure substance. It follows, that the water of the chemist is one thing, the water of nature another. The ideal water of the chemist is a single substance; the water of nature, like the air of nature, is a mixture of substances. Just as this last is a variable mixture of the pure chemical substances -nitrogen, oxygen, water vapour, carbonic acid and ammonia, with traces of various other kinds of matter, so is the water of nature a variable mixture of the pure chemical substances-protoxide of hydrogen, common salt, saltpetre, gypsum, limestone, carbonic acid, nitrogen, and oxygen, &c., together with various kinds of organic matter.

Air, from which the minor constituents of atmospheric air have been carefully abstracted, is sometimes spoken of as purified air; but air, deprived in this way of its so-called impurities, is absolutely incapable, in relation alike to animal and vegetable life, of fulfilling the functions of an atmosphere. The substitution of such purified air for actual atmospheric air would mean the cessation on the earth's surface of all life as it now exists. Similarly, with regard to natural water, some of its minor constituents we know to be essential, others of them we have reason to think advantageous to the fulfilment of its functions in nature. Bearing in mind, indeed, the interaction everywhere of life and the conditions of living, we can scarcely doubt that the actual mixed substance, water, is better suited to supply the wants of our daily life, than the ideal unmixed substance would be; and further, we have no reason whatever to look upon this ideal substance as furnishing a standard of excellence, to which it is desirable that our daily supply should, as far as practicable, and in all respects, approximate. What we really desiderate is not chemical purity, but hygienic freedom from anything hurtful. In some water, as in some air, an objectionable constituent may be met with; but in the case neither of water nor air, is there any presumption on the score of wholesomeness, in favour of a single substance as such, rather than of a mixed substance as such. The presumption is indeed the other way. No one desires unmixed oxygen, or even oxygen and nitrogen free from commixture with the minor constituents of our native air; so neither is there any reason to desire unmixed protoxide of hydrogen, freed from the minor constituents of wholesome natural water. It is clearly open to a chemist, addressing himself to chemists, to speak of the constituents of ordinary water, other than protoxide of hydrogen, as impurities in the water; provided of course that he is consistent, and includes the desirable dissolved gases of the water among its impurities and as contributories to the sum total of its impurity. It is also open to a chemist, addressing himself to chemists, to make

a comparison of different natural waters, in respect to the relative proportions of their total impurity, or of their saline impurity, or of their calcareous impurity, or of their organic impurity, or of their aerial impurity, &c. But it is not, I take it, open to a chemist addressing the general public, to speak as a chemist of some particular selected constituent, characteristic of the class of natural waters derived from one kind of source, as an impurity, and to leave it to be inferred by the general public that because this constituent is, in a strict chemical sense, an impurity, it is therefore a something nasty and unwholesome, and that the class of waters in which it is more especially met with are, in proportion to the extent of its presence, nasty and unwholesome. The general public do not know that the chemical impurity of a water may be good or bad, noxious or innoxious, desirable or undesirable, wholesome or unwholesome. They are unaware that no scale of wholesomeness or desirableness can be inferred from a scale of chemical purity, with respect to some particular constituent stigmatised as an impurity, until it has been established by evidence that this particular chemical impurity is of an unwholesome or prejudicial character. It is a mere dialectic artifice, and not a very worthy artifice, to use the word impurity in a strictly scientific sense, with intention to have it accepted in a popular sense as bearing a meaning which, scientifically, by no means belongs to it.

3.-ORGANIC MATTER OF WATER.

The saline matter of water is a useful expression to denote the sum of the dissolved mineral constituents, and the organic matter of water a useful expression to denote the sum of the dissolved organic constituents of natural water. The saline matter of one water is not necessarily, or yet commonly, identical with the saline matter of another water; nor is the saline matter of the selfsame body of water necessarily identical throughout its entire extent, or from one period of time to another. The same holds good with regard to the organic matter of water. It

is a different thing in one water from what it is in another; and may be different in the same water from place to place and from time to time. But just as there is a character or nature more or less common to the usual varieties of dissolved saline matter, so also is there a character or nature common, within certain limits, to the usual varieties of organic matter met with in potable water. So far a certain parallelism holds good between the saline matter of water on the one hand, and its dissolved organic matter on the other. But in many important particulars the parallelism fails. Thus the saline matter occurs, for the most part, in appreciable proportion, say from to of a per cent. It is constituted of definite chemical substances, possessed of well-determined properties; and its amount is capable of estimation with all desirable accuracy—with considerable accuracy by direct weighing, and with yet greater accuracy indirectly, by the separate estimation of its several constituents. The dissolved organic matter of potable water on the other hand, never amounts to more than an exceedingly minute proportion, say from to Too of a per cent.; what are its separate constituents, and what their chemical nature, is almost, if not wholly, unknown; its amount is moreover incapable of direct determination, and the different means for its indirect determination are far from satisfactory. Chemists are capable, however, thanks to Dr. Frankland, of determining with a considerable degree of accuracy, the quantity of carbon that exists in any water, in the form of organic matter. Evidently, if all organic matter contained the same proportion of carbon, a determination of the organic carbon present in any water would be tantamount to a determination of its organic matter. But in reality the proportion of carbon existing in different individual varieties of organic matter has, on the contrary, a very wide range of variation. Still, it would appear, from such imperfect investigations as have been made, that the proportion of carbon existing in the organic matter of waterthat is to say, the mean proportion of carbon present in

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