The proportions of free and albuminoid ammonia found in the preceding operations are usually expressed in parts per 100,000 of the water. Wanklyn gives the following amounts of free and albuminoid ammonia contained in 100,000 parts of several kinds of water:-- Deep spring water not over 0·001 „ „ „ mixed with surface water 0·005 Filtered water 0·005 to 0·010 Imperfectly filtered water 0·01 „ 0·02 Sewage 0·30 The same authority makes the following classification of potable water, reference being made to parts of albuminoid ammonia present in 100,000 parts:-- Extraordinary purity 0 to 0·005 Satisfactory purity 0·005 „ 0·010 Dirty over 0·010 The presence of any considerable proportion of free ammonia is usually indicative of recent sewage contamination. In the absence of free ammonia, a water need not be rejected unless the albuminoid ammonia exceeds 0·010 part, but a water containing over 0·015 part of albuminoid ammonia should be condemned under all circumstances. 6. _Nitrogen as nitrites and nitrates._--It is quite generally accepted that the presence in water of the oxidation products of nitrogen, is to be ascribed to the oxidation of nitrogenous organic matter, unless they are the result of percolation through soil containing nitrates, and, for this reason, considerable importance attaches to the quantitative estimation of the nitrogen present in the state of nitrates, and, in some cases, nitrites. One of the most reliable methods for this determination is the eudiometric process of Frankland, which is based upon that of Crum,[125] and consists in agitating the concentrated water with mercury and strong sulphuric acid, and measuring the volume of nitric oxide formed by the reduction of nitrates and nitrites. Owing, however, to the necessity of employing gas apparatus, this method is not in very general use. Wanklyn’s process is the following:--100 c.c. of the sample are made alkaline with pure sodium hydroxide, evaporated to about one-fourth of its original volume, next made up to 100 c.c. by adding pure distilled water, and introduced into a flask which connects with a U-tube filled with powdered glass moistened with hydrochloric acid. A piece of aluminium foil is then added to the contents of the flask, and the mixture is allowed to stand at rest for six or seven hours. The contents of the U-tube are now transferred to the flask, the latter is connected with a Liebig’s condenser and the liquid distilled. The proportion of ammonia contained in the distillate is determined by Nessler’s reagent as previously described, from which the amount of nitrogen present as nitrates and nitrites is calculated.