3. _Organic matter in solution._--A method frequently employed for this determination is based upon the supposition that the amount of potassium permanganate required to oxidise the organic constituents contained in water would serve as a criterion of its sanitary value. It is generally known as the “Forchammer” or “oxygen” process, and, although of undoubted service in comparing the quality of samples of very impure water, it is defective in the following important respects: Different organic substances are not affected to an equal extent by potassium permanganate; albumen, for instance, being far less easily oxidised than other compounds, and the value of the results afforded is vitiated by the presence of certain inorganic bodies, such as nitrites, sulphuretted hydrogen, ferrous salts, etc. It has been stated, that the more deleterious and putrescent organic ingredients of water are those most readily affected by the permanganate solution. As modified and improved by Miller[121] and by Tidy,[122] the process consists substantially in adding an excess of a standard solution of potassium permanganate to a measured quantity of the water under examination (acidulated with sulphuric acid), and then determining the excess of permanganate used by means of sodium hyposulphite and potassium iodide. The following solutions are required:-- _Potassium Permanganate._--0·395 gramme of the salt is dissolved in 1 litre of distilled water; 10 c.c. of this solution represent 0·001 gramme of available oxygen. _Sodium Hyposulphite._--One gramme of the salt is dissolved in a litre of water. _Starch solution._--One gramme of starch is triturated with about 20 c.c. of boiling water, and the mixture allowed to stand at rest over night, after which the clear supernatant solution is drawn off. _Pure distilled Water._--This is prepared by digesting 10 litres of distilled water with 10 grammes of potassium hydroxide and 2 grammes of potassium permanganate in a still provided with an inverted condenser at 100° for twenty-four hours, after which the water is distilled, separate portions being frequently tested with Nessler’s solution; the distillate is not reserved for use until this reagent ceases to produce a brownish coloration.