The determination of such pollution is a matter of great delicacy and difficulty, and can only be trusted to an expert. Waters polluted by organic matters often contain an excess of gaseous constituents, and are clear, sparkling, and palatable, presenting to the uninstructed eye no indication of impurity. There are certain tests, however, which can be used by any person of intelligence, when, if positive results are obtained, an expert should be called in to determine the source and character of the contamination. The pollution of water by decomposing animal matters is always to be suspected, if there are evidences of the presence of chlorine or nitrogen in the water, as these are invariable constituents of animal excreta. These substances are found in combination—the former in chloride of sodium, and the latter in the so-called nitrites and nitrates.[2] Their presence is determined as follows: [2] The albuminoid ammonia test is too technical for insertion here. =Chlorine.=—_Tests_: Nitrate of silver (twenty-five cents a drachm), twenty-four grains to one ounce of distilled water, and dilute nitric acid (ten cents an ounce). Pour a few drops of each into the suspected water. If chlorine is present, there will be a cloudy-white precipitate of chloride of silver, which will gradually turn darker. One grain of chlorine to a gallon of water gives a haze; four grains a marked turbidity, and ten grains a considerable precipitate. In case chlorine is found, and any particular source is suspected, a pailful of salt (chloride of sodium) and water may be thrown into the place from which the leakage is supposed to come, and the water again examined, after a few hours, to see whether the amount of chlorine has increased. =Nitrates.=—_Tests_: Pure sulphuric acid and a saturated solution of sulphate of iron (copperas). Add an equal bulk of the acid to any quantity of the water in a test-tube. The mixture will become very hot. Wait until it is cool, and then pour in the iron solution gently, so that it will float above the mixed acid and water. If nitrates are present, there will be an olive-colored layer where the fluids meet. =Nitrites.=—_Test-mixture_: Iodide of potassium (fifty cents per ounce), one part; starch, twenty parts; water, five hundred parts. Make the starch-solution first, and filter when cold; then add the iodide of potash. Add to the suspected water this mixture, and then a little dilute sulphuric acid. If nitrites are present, there will be an immediate blue color.