This test is employed for ascertaining the presence of earthy salts in waters. Hence it produces no change when mingled with distilled or perfectly pure water; but when added to water containing earthy salts, a white flocculent matter becomes separated, which speedily collects on the surface of the fluid. Now, from the quantity of flocculent matter produced, in equal quantities of water submitted to the test, a tolerable notion may be formed of the degrees of hardness of different kinds of water, at least so far as regards the fitness of the water for the ordinary purposes of domestic economy. This may be rendered obvious in the following manner. EXPERIMENT. Fill a number of wine-glasses with different kinds of pump or well water, and let fall into each glass a few drops of the solution of soap in alcohol. A turbidness will instantly ensue, and a flocculent matter collect on the surface of the fluid, if the mixture be left undisturbed. The quantity of flocculent matter will be in the ratio of the quantity of earthy salts contained in the water. It is obvious that the action of this test is not discriminative, with regard to the chemical nature of the earthy salt present in the water. It serves only to indicate the _presence_ or _absence_ of those kinds of substances which occasion that quality in water which is usually called _hardness_, and which is always owing to salts with an earthy base. If we wish to know the nature of the different acids and earths contained in the water, the following test may be employed.[15] EXPERIMENT. Add about twenty drops of a solution of oxalate of ammonia, to half a wine-glass of the water; if a white precipitate ensues, we conclude that the water contains lime. By means of this test, one grain of lime may be detected in 24,250 of water. If this test occasion a white precipitate in water taken fresh from the pump or spring, and not after the water has been boiled and suffered to grow cold, the lime is dissolved in the water by an excess of carbonic acid; and if it continues to produce a precipitate in the water which has been concentrated by boiling, we then are sure that the lime is combined with a fixed acid. EXPERIMENT. To detect the presence of iron, add to a wine-glassful of the water a few drops of an infusion of nut-galls; or better, suffer a nut-gall to be suspended in it for twenty-four hours, which will cause the water to acquire a blueish black colour, if iron be present. EXPERIMENT. Add a few grains of muriate of barytes, to half a wine-glass of the water to be examined; if it produces a turbidness which does not disappear by the admixture of a few drops of muriatic acid, the presence of sulphuric acid is rendered obvious. EXPERIMENT.