Griess[126] has suggested a very useful process for the determination of nitrous acid and nitrites in potable waters. It is executed by placing 100 c.c. of the filtered water in a glass cylinder, and adding a few drops of dilute hydrochloric acid, and 1 c.c. of a solution of sulphanilic acid and naphthylamine hydrochloride. In the presence of nitrites, a beautiful rose-red colour (due to the formation of azobenzol-naphthylamine sulphonic acid), will be produced. The proportion of nitrites contained in the water, is ascertained by simultaneously subjecting a solution of potassium nitrite, of known strength, to the same treatment, and matching the degree of colour obtained, as in the Nessler process. This solution can be prepared by dissolving 0·406 gramme of dry silver nitrite in hot water, and adding a slight excess of potassium chloride. After cooling, the solution is made up to one litre, the silver chloride allowed to settle, and the clear liquid filtered. If 100 c.c. of the filtrate are further diluted to one litre, each c.c. will contain 0·00001 gramme of nitrous acid. In Ditmar’s method, the residue obtained by the evaporation of the water, is first mixed with pure sodium hydroxide, and placed in a small silver boat. It is next introduced into a combustion tube and burned in a current of hydrogen, the evolved gases being received in an absorption apparatus filled with very dilute hydrochloric acid. In this method the amount of ammonia formed, is likewise estimated by means of Nessler’s solution. The proportion of _organic nitrogen_ is found by deducting the free ammonia present in the water and multiplying the remainder by 14/17. Messrs. Dupré and Hake[127] determine the _organic carbon_ in water essentially as follows:--The residue of the evaporation of the water is obtained in a very thin silver dish, which can be rolled up and introduced into a combustion tube filled three-fourths of its length with cupric oxide. The residue is then burned in a stream of oxygen. The evolved carbonic acid is absorbed in a solution of barium hydroxide, the precipitate formed being collected upon a filter, washed, dried, and weighed; its weight, divided by 19·4, gives the amount of organic carbon present in the sample. The carbonates and nitrates originally contained in the water can be removed by boiling with a saturated solution of sulphurous acid before the preliminary evaporation. Frankland gives the following average proportions of nitrogen, as nitrates, occurring in 100,000 parts of various kinds of water:-- Rain water 0·007 Upland surface water 0·009 Deep wells and springs 0·400 Surface water (cultivated districts) 0·250 Shallow wells (no average), 2 to 5 parts common. Other authorities regard the presence of more than 0·6 part of nitrogen as nitrates per 100,000 parts of water as indicating dangerous pollution.