Many tests have been proposed, of a chemical and organoleptic character, as well as the color tests, for judging of the purity of water and as a means of classifying the water as bad. All of these tests have more or less value, without doubt, but none of them, alone, can be considered as authoritative. If water is colored, it is evidently due to contamination, which ought to be avoided. Many persons consider bog water, which has a distinctly brown tint, as not being thereby disqualified for drinking or other purposes. Water which is colored by the refuse of industrial operations can hardly be regarded as suitable for human consumption, even if the coloring agents themselves be not distinctly of a poisonous or deleterious character. In other words, the inorganic coloring matters, to which the color of water may be due, while less open to suspicion and more readily identified, are not for that reason to be classed other than the organic coloring matters, which the water may extract from the soil through which or over which it passes.