If the outward appearance of water, clearness, brightness, and tastelessness, were a reliable criterion of the purity of the water, nothing would be easier than to form a fair estimate of its value; but this is by no means the case. Brightness is only a proof of a perfect absence of suspended impurities, but it gives no evidence of matters not being present in solution. One-quarter of a grain of chalk in suspension in a gallon of pure water will suffice to give the whole a dull and turbid appearance; while a hundred times the quantity in solution leaves the water limpid and perfectly transparent. So it is with innumerable other substances that may be contained in water, without in any way manifesting their presence, except by the agency of chemical re-agents.

Generally, polluted waters have various shades of a yellowish or brownish tint, which vary according to the amount of filth which they contain; but to this there are so many exceptions, that the color is by no means a safe guide. Some peaty waters, and those that contain iron may have a yellowish or brownish tint, and yet be perfectly healthy. On the other hand, some very badly polluted waters are perfectly clear, and frequently present a better appearance than many pure waters. Good water should have practically no color, though a slight tinge may be present in an otherwise excellent water.

If turbidity arises from sand or clay the water will rapidly become clear on standing; but if it arises from organic matter, this is not generally the case and is an unfavorable sign.

The character of a water can seldom be determined from any one indication or test. The accumulated evidence of a number of tests is necessary for the formation of a correct opinion of its quality. Occasionally, from the most accurate and numerous tests that can be made in a fully equipped laboratory, it is impossible to pronounce on some waters, while others are so marked in character that a few tests declare at once what they are.

The smell of a water often gives some indication of its character. But it frequently happens that wholesome waters have an unpleasant odor: this is the case with some mineral waters. In clayey districts, especially, water which is organically pure may have an objectionable odor, which is imparted by the clay. The waters of some lakes and rivers which supply some of our large cities, as Boston, New York, and Baltimore, have at times a peculiar "fish-like " odor. It generally begins in summer, but sometimes not until autumn. It is due, probably, to some condition of water plants - whether to a state of growth, or decay, is uncertain. Growing plants emit odors peculiar to themselves: so it is not necessary to suppose that the odor mentioned arises from decay. However it may be, there is yet no evidence that such water is injurious to the health of those who drink it.

If the odor is very marked, of course there is no difficulty in perceiving it; when this is not the case, partly fill a clean bottle with the water to be tested, and after shaking it violently, so as to communicate the odor to the air within the bottle, place it in a kettle of cold water, and heat the whole together, or, after strong agitation, inhale the air of the bottle through the nostrils. Heat expels the gases dissolved in the water, so that they may be detected on removing the stopper. Finally, the odor may be made more apparent by adding a little caustic potash to the water.

Pure water has no odor whatever, and should be tasteless; but water may even be tasteless and yet be very bad.

It is very desirable that, besides examining a water in its perfectly fresh condition, samples of it should be set aside, in half-filled but close glass-stoppered bottles, for some time - say 10 or 12 days - and one of these examined every day or two, so as to trace the character and extent of the changes undergone. Not only may conclusions be drawn from such a series of observations as to the general stability or decomposability of the organic matter present, but light will be thrown upon the changes which may be expected to occur under ordinary conditions when the water is stored for use, as in cisterns, wells during periods of drought, or carelessly allowed to remain stagnant in pitchers, water coolers, etc.

The following few simple tests are suggestive:

Color

Fill a bottle made of colorless glass; look through the water at some black and, following, at some white object; the water should appear perfectly colorless. A muddy or turbid appearance indicates the presence of soluble organic matter, or of solid matter in suspension.

Turbidity

This may arise from sand, clay, or from organic matter. If from either of the former two, the water will rapidly become clear on standing; but if the turbidity arise from organic matter this is not generally the case and is an unfavorable sign. If a microscope is accessible and living organisms can be seen, the water should be decidedly rejected. The quantity of suspended matter is ascertained by filtering through a filter previously weighed. After filtration dry filter, and filtrate and weigh again. The difference in weight indicates the quantity of suspended matter.

Odor

Fill and cork a bottle with some of the suspected water, and place it for a few hours in a warm place; shake it, remove the cork, and if the odor is in the least repulsive, the water should be rejected. By heating the water to boiling, an odor is evolved that otherwise does not appear.

Taste

Water fresh from a well is usually tasteless, even though it may contain a large amount of putrescible organic matter, thus rendering this test no criterion as to the quality of a water. Water for use should be perfectly tasteless, and remain so even lifter it has been warmed.