If an 8 oz. bottle shows turbidity, the presence of more than 8 volumes per 10,000 is indicated; how much more must be determined by a second experiment. Taking a 6½ oz. bottle, the air is known to contain less than 10 volumes if no precipitate is developed. The carbonic acid can then be stated as constituting from 8 to 10 volumes per 10,000 of the air. But a third experiment with a bottle intermediate in size will correspondingly reduce the limits of uncertainty regarding the carbonic acid figure. There is no test-paper which can be made practically useful as a quantitative test for carbonic acid. (Sanitary Engineer.)
Water.—The tests for water embrace impurities which affect the character of water for drinking, cooking, and washing purposes. Drinking-water should not be too soft, as it provides much of the lime required in building up the bones of the body; the chief evil in drinking-water is the presence of organic ferments. For cooking and washing purposes, water cannot be too soft, and, if used boiled, the presence of organic matters is practically neutralised.
Drinking-water.—In 1871, Dr. Hager published a valuable and simple test for the presence of fermentable poisonous matter. He proposed a tablespoonful of a clear solution of tannin to be added to a tumblerful of water. If no gelatinous turbidity occurs within 5 hours, the water may be considered good. If turbidity occurs within the first hour, the water is unwholesome. If turbidity is displayed within the second hour, the water is not to be recommended. Previously, in 1866, Dr. Hager had recommended for travellers, as a precaution in cholera times, the addition of the following solution (20 drops to 1 pint) to any water they might be about to drink:—Tannic acid, 5 parts; syrup, 4 parts; distilled water, 6 parts; spirit of wine, 12½ parts.
A very simple test for the purity of water is given by Heisch. He observes that good water should be free from colour, and unpleasant odour and flavour, and should quickly afford a good lather with a small proportion of soap. If ½ pint of water be placed in a clean colourless glass-stoppered bottle, a few grains of the best white lump sugar added, and the bottle freely exposed to the daylight in the window of a warm room, the liquid should not become turbid, even after exposure for a week or 10 days. If, while the stopper remains secure, the water becomes turbid, it is open to grave suspicion of sewage contamination; but if it remains clear, it is almost certainly safe for drinking and all domestic purposes.
Hard or Soft Water.—Dissolve a small quantity of good soap in alcohol. Let a few drops fall into a glass of water. If it turns milky, it is hard; if not, it is soft.
Earthy Matters or Alkali.—Take litmus paper dipped in vinegar, and if, on immersion, the paper returns to its true shade, the water does not contain earthy matter or alkali. If a few drops of syrup be added to a water containing an earthy matter, it will turn green.
Carbonic Acid.—Take equal parts of water and clear lime water. If combined or free carbonic acid is present, a precipitate is seen, in which, if a few drops of muriatic acid be added, an effervescence commences.
Magnesia.—Boil the water to 1/20 part of its weight, and then drop a few grains of neutral carbonate of ammonia into a glass of it, and a few drops of phosphate of soda. If magnesia be present, it will fall to the bottom.
Iron.—(a) Boil a little nut-gall, and add to the water. If it turns grey or slate colour, iron is present. (b) Dissolve a little prussiate of potash, and if iron is present, it will turn blue.