In the blank experiment 20 c.c. ferrous ammonium sulphate solution required 9.8 c.c. one-tenth strength bichromate solution, so that 20.4 c.c. ferrous solution would equal 10 c.c. bichromate. 20.4 c.c. ferrous solution required 27.8 c.c. of oxidised glycerine solution before it ceased to give a blue coloration with potassium ferricyanide. {0.25 - (250 x 0.01)} Therefore, per cent. of glycerol = { ------------} x 500 { 27.8 } = 80.04 per cent. Other methods have been suggested for the preliminary purification, _e.g._, silver oxide, silver carbonate and lead subacetate, and copper sulphate and caustic potash, but the lead subacetate alone with care gives satisfactory results. Other determinations include those of specific gravity, alkalinity, proportion of salts and chloride, and tests for metals, arsenic, sulphur compounds, sugar, and fatty acids. _Specific gravity_ is determined at 15 deg. C., and may be taken in specific gravity bottle, or with a Westphal balance or hydrometer It usually ranges from 1.3 to 1.31. _Alkalinity_, which is usually sodium carbonate, and may be somewhat considerable if the soap has been grained with caustic alkali, is determined after dilution with water by titrating with N/2 acid, using methyl orange as indicator. _Salts._--These may be determined by gently incinerating 5-6 grammes of the glycerine, extracting the carbonaceous mass with distilled water, filtering, and evaporating the filtrate on the water bath. The dried residue represents the salts in the weight taken. _Chloride of sodium_ (common salt) may be estimated by dissolving the total salts in water, adding potassium chromate, and titrating with N/10 silver nitrate solution. _Copper_, _lead_, _iron_, _magnesium_, and _calcium_ may also be tested for in the salts, by ordinary reactions. _Arsenic_ is best tested for by the Gutzeit method. About 5 c.c. is placed in a test-tube, a few fragments of granulated zinc free from arsenic, and 10 c.c. dilute hydrochloric acid added, and the mouth of the tube covered with a small filter paper, moistened three successive times with an alcoholic solution of mercury bichloride and dried. After thirty minutes the filter paper is examined, when a yellow stain will be observed if arsenic is present. _Sulphates._--These may be precipitated with barium chloride in acid solution, in the usual way, dried, ignited, and weighed. _Sulphites_ give with barium chloride a precipitate soluble in hydrochloric acid. If the precipitate is well washed with hot water, and a few drops of iodine solution together with starch paste added, the presence of sulphites is proved by the gradual disappearance of the blue starch-iodine compound first formed.