As water is a solvent of nearly all saline matter, in a larger or smaller degree, and of liquid and elastic fluids, with but few exceptions, and necessarily comes in contact with some or others of them, it is evident that water must be expected to retain some, however small a proportion, of one or more of them. The affinity of many chemical elements and compounds to water is so intense and difficult to overcome, that it remains indeed doubtful if, by the most careful distillation or whatever other methods employed, water can be successfully freed from a last trace of extraneous matter, not to be discernible by methods more perfect than those yet employed.

It will, however, be a comfort to know that few industries - not to mention domestic uses - require water even as pure as distilled; and that for many of our largest industrial operations a considerable proportion of foreign admixture of some kind in the water is quite harmless, though a much smaller proportion of another kind may unfit it for the same purpose, while for another purpose the reverse may be the case.

For instance, potable water may contain a comparatively large proportion of some saline matter without injury, and should contain oxygen or carbonic acid, or both, while less than one-hundredth part as much of organic ammonia entirely disqualifies it. Or, to mention one other case, the water used by the great Burton (English) brewers, and superior to almost any other known for the purpose, contains over sixty-five grains of mineral matter in the gallon, principally carbonate and sulphate of lime, with considerable (10.12) of chloride of sodium or common salt, 9.95 of sulphate of magnesia, and 7.65 sulphate of potassa, with some other salts, while the.same water is quite unfit for various other purposes.

* On this subject and the Aeration of Water we are indebted to Mr. d'Heureuse, New York.

This shows the importance of ascertaining the quantities as well as the qualities of extraneous matter in the water to determine its suitability for any intended purpose, besides various mineral or organic substances which natural water may, and invariably does hold in solution. It generally carries in suspension particles of insoluble matter by which its transparency or brilliancy is impaired in proportion to the quantity and kind of the admixture. This floating or sedimentary matter is frequently of a mineral character, like clay or earthy matter carried by turbid streams at times of freshets, or may consist of organic substance, partly decomposed animal or vegetable matter, living or dead organisms of microscopic or larger size, and other fragmentary substances.

If the purity of the water depended solely upon the absence of this insoluble matter suspended in it, the problem of obtaining pure water would be reduced to the simple mechanical operation of filtering the water, as suitable filters of various designs are plentiful to retain the suspended or sedimentary matter and yield brilliantly clear from a turbid or muddy water. But, unfortunately, water thus rendered bright and brilliant, is by no means certain to be pure in its true meaning and purpose; in fact, it is generally but little purer than before. The tangible gross admixtures, visible to the eye, but possibly harmless for the purpose intended, have been removed by the filter, while the more intangible, but in a much smaller proportion, deleterious impurity can remain unchanged; and the popular error, confounding brilliancy with purity of water, sacrifices many valuable lives annually.

From the foregoing it will be evident that, where water plays as prominent a part as in the line of the mineral water trade, and business success largely depends upon the proper quality of the water employed, it is essential to ascertain the composition of the water and its possible defects, with the view of changing the supply or correct what we have.

As the proportions of extraneous matter in the natural fresh-water supply are always comparatively small, the determination of the kinds and quantities of components is obviously not in everybody's reach, and to obtain anything like reliable results such determination must be left to those qualified to the task by knowledge of chemistry and the use of the delicate instruments employed.

Of course it is easy enough, though somewhat slow and tedious work, to carefully evaporate from one-half to two gallons of water down to dryness in a large, clean platina dish, the weight of which has been previously carefully determined, and which is weighed again after the complete evaporation, the increase in weight indicating that of the total solid sub-stances contained in the quantity of water so treated. But the fact that, for instance, 2 1/2 to 53 or 107 grains of solid matter are in the gallon of water conveys but a very imperfect idea as to the suitability of the water for many purposes, and though the only direct result generally obtained hardly permits per se many correct conclusions. All other tests employed to ascertain if and how much of a certain substance or compound is contained in the water are indirect, and the result of conclusions after the following fashion: that, if a stated amount of a certain re-agent added to the liquid precipitates an ascertained weight of a compound formed by addition of the re-agent, a calculable amount of another compound (say of lime or magnesia) is proved to have been contained in a known quantity of the water. Or again, by the addition of some re-agent to a known quantity of the water, possibly previously subjected to distillation or some other operation, a more or less intense color appears, the degree of intensity of which is positive proof to the expert that a certain proportion of, say ammonia, is in a gallon of the water, though only a fraction of the gallon was submitted to the test. The degree of hardness of water is easily established by the proportion of soap required to overcome it; but even this requires some care to allow approximately correct conclusions.