_Procedure._—Place a measured portion of the sample, or the proper amount of the 1 per cent stock suspension of the sample, in a 300 cc. narrow-mouth glass-stoppered bottle, and dilute it to the desired concentration with water saturated with oxygen. Determine the oxygen content at 20° C. of the waters that are used for dilution. This determination must be made before the mud or sludge is added because iron sulfide in the mud or sludge rapidly consumes part of the dissolved oxygen. Incubate at 20° C. for five days. Shortly before the determination of the oxygen remaining in solution at the end of five days rotate the bottle once or twice to mix its contents and allow sedimentation for about 30 minutes. Siphon the greater part of the liquid through a narrow-bore siphon into a 150 cc. bottle, which has been filled with carbon dioxide. Reject the first 25 cc. of the siphoned liquid and allow a little to overflow at the end of siphoning. Determine the oxygen content of the solution in the bottle in the usual way (pp. 65–68). Report the oxygen demand in percentage of the dried mud or sludge. ANALYSIS OF CHEMICALS. The following sections describe the accepted methods for the analysis of the chemicals commonly used in the treatment of water. REAGENTS. 1. Distilled water. In practically all the tests of chemicals it is necessary to use exclusively distilled water that has been freshly boiled to free it from carbon dioxide and oxygen. 2. Concentrated hydrochloric acid. Sp. gr. 1.20. 3. Hydrochloric acid, N/2. 4. Hydrochloric acid, N/10. 5. Ammonium hydroxide. Redistilled; sp. gr. 0.90. 6. Dilute sulfuric acid. Dilute 1 part of concentrated sulfuric acid with 3 parts of freshly boiled distilled water. 7. Methyl orange indicator. See page 36. 8. Phenolphthalein indicator. See page 36. 9. Bromine water. 10. Stannous chloride, N/20. This should be frequently standardized by titration against a standard iron solution. One cc. of N/20 stannous chloride is equal to 0.0028 gram of iron (Fe) estimated in the ferrous state. 11. Sodium hydroxide, N/1. Free from carbonate. This should be frequently standardized by titration against a standard acid solution in presence of phenolphthalein indicator. One cc. of N/1 sodium hydroxide is equal to 0.049 gram of sulfuric acid (H_{2}SO_{4}), or to 0.03645 gram of hydrochloric acid (HCl). 12. Sodium hydroxide, N/20. Free from carbonate. 13. Standard potassium permanganate. A N/10 solution. One cc. of N/10 potassium permanganate is equal to 0.0056 gram of iron (Fe) estimated in the ferrous state. 14. Alcohol. Ethyl alcohol, 95 per cent. 15. Sugar. Solid granulated cane sugar. SULFATE OF ALUMINIUM.