The determination proper is executed as follows:--Two flasks are first thoroughly cleansed by washing with concentrated sulphuric acid, and subsequently with water; 250 c.c. of the water to be examined are introduced into one, and the same volume of the pure distilled water, prepared as above, is placed in the other. 10 c.c. of dilute sulphuric acid (1 part pure acid and 8 parts distilled water) and 10 c.c. of the potassium permanganate solution are now added to each flask, both then being put aside for three hours. Two drops of a 10 per cent. solution of potassium iodide are next added to the flasks, and the amount of iodine liberated (which is equivalent to the quantity of permanganate unacted upon by the water) is determined by titration with the sodium hyposulphite solution. The precise end of the reaction is ascertained by means of a few drops of the starch paste, the hyposulphite being added to each flask until the blue colour produced by the starch disappears. The quantities of solution used in each titration are then read off. The amount of permanganate consumed is equal to A-B, where A represents the hyposulphite used with the distilled water, and B, that used with the sample under examination, and the proportion of oxygen which is consumed by the water tested, can be calculated by the formula:-- ((A - B) _a_) / A in which _a_ is the available oxygen in the added permanganate. For example, if 10 c.c. of permanganate (= 0·001 gramme available oxygen) are added to the 250 c.c. (= ¼ litre) contained in each flask, and the distilled water required 35 c.c., the sample 15 c.c., of the hyposulphite solution, the proportion of oxygen consumed by the ¼ litre of water, would be ((35 - 15) × ·001) / 35 = ·000571, which represents ·228 parts of oxygen in 100,000 parts of water. In applying the preceding test, it is requisite that the flasks should be kept at a particular temperature, such as 27°. The presence of putrescent and readily oxidised organic matter or nitrites, which indicates dangerous contamination, is recognised by the absorption of any considerable proportion of oxygen in the space of two minutes. According to Dr. Tidy, 100,000 parts of water of various degrees of purity, absorb the following amount of oxygen in three hours:-- Part Oxygen. 1. Great organic purity 0 to 0·05 2. Medium purity 0·05 „ 0·15 3. Doubtful 0·15 „ 0·21 4. Impure over 0·21