Ignite, at dull red heat in a hood, the residue from the determination of moisture until all the carbon has disappeared. Cool the residue in a desiccator and weigh it. The residue is the fixed matter. The volatile matter is the difference in weight between the original dried sludge and the ignited sludge. TOTAL ORGANIC NITROGEN. _Preparation of sample._—For the determination of organic nitrogen and fats dry approximately 50 to 75 grams of the sludge or mud in a porcelain dish first on the water bath and finally in the hot-water oven until all the moisture has disappeared. Grind the dry material to a fine powder and keep it in a glass-stoppered bottle. _Reagents._—1. Sulfuric acid. Concentrated, nitrogen-free. 2. Copper sulfate solution. Ten per cent. 3. Potassium permanganate. Crystals. _First procedure._—Weigh accurately 0.5 gram of dried sludge or 5.0 grams of dried mud and put it into a 500 cc. Kjeldahl flask. Digest it with 20 cc. of sulfuric acid, or more if necessary, and 1 cc. of copper sulfate solution to assist the oxidation. Boil for several hours until the liquid becomes colorless or slightly yellow. Oxidize the residue with 0.5 gram of potassium permanganate, and follow the “Procedure for Sewage” (pp. 21–22). _Second procedure._—The following method is convenient for routine work at sewage disposal plants. After digestion as described in the first procedure, cool, transfer to a glass-stoppered 100 cc. flask, dilute with distilled water to 100 cc., and mix well. Transfer 50 cc. with a pipette into another 100 cc. volumetric flask, and make this portion alkaline with 50 per cent sodium hydroxide, testing a drop of the liquid on a porcelain plate with phenolphthalein to insure neutralization. The formation of a floc usually indicates that neutralization is complete. Dilute the solution to 100 cc., pour it into a small glass-stoppered bottle and permit it to stand until the next day. Nesslerize an aliquot portion of the clear supernatant liquid, and calculate the percentage of nitrogen in the material. ETHER-SOLUBLE MATTER. Fats are usually determined only on sewage sludge, but some mud deposits contain small quantities due to the presence of trade wastes.