=176. The Color of Water.=—The potability[7] of water collected from any watershed is materially affected by its color. Although iron may produce a brownish tinge, by far the greater amount of color is produced by dissolved vegetable matter. Repeated examinations of colored water have shown that discoloration is in many cases at least a measure of the vegetable matter contained in it. While this may not indicate that the water is materially unwholesome, it shows conclusively the existence of conditions which are usually productive of minute lower forms of vegetation from which both bad taste and odors are likely to arise. [7] What is generally known as the “Michigan standard of the purity of drinking-water,” as specified by the Michigan State Laboratory of Hygiene, is here given: “1. The total residue should not exceed 500 parts per million. “2. The inorganic residue may constitute the total residue. “3. The smaller amount of organic residue the better the water. “4. The amount of earthy bases should not exceed 200 parts per million. “5. The amount of sodium chloride should not exceed 20 parts per million (i.e., ‘chlorine’ 12.1 parts per million). “6. The amount of sulphates should not exceed 100 parts per million. “7. The organic matter in 1,000,000 parts of the water should not reduce more than 8 parts of potassium permanganate (i.e., ‘required oxygen’ 2.2 parts per million). “8. The amount of free ammonia should not exceed 0.05 part per million. “9. The amount of albuminoid ammonia should not exceed 0.15 part per million. “10. The amount of nitric acid should not exceed 3.5 parts per million (i.e., ‘N as nitrate’ .9 part per million). “11. The best water contains no nitrous acid, and any water which contains this substance in quantity sufficient to be estimated should not be regarded as a safe drinking-water. “12. The water must contain no toxicogenic germs as demonstrated by tests upon animals. “The water must be clear and transparent, free from smell, and without either alkaline or acid taste, and not above 5 French standard of hardness.” This standard is too high to be attained ordinarily in natural waters. There are two periods in the year of maximum intensity of color, one occurring in June and the other in November. The former is due to the abundant drainage of peaty or other excessively vegetable soils from the spring rains. After June the sun bleaches the water to a material extent until the autumn, when the dying vegetation imparts more or less coloring to the water falling upon it. This last agency produces its maximum effect in the month of November.