After the lye has been filtered to the filtered lye tank it is fed to the evaporator, the method of operation of which varies somewhat with different styles or makes. When it first enters the evaporator the lye is about 11deg.-12deg. B. After boiling the density will gradually rise to 27deg. B. and remain at this gravity for some time and during which time most of the salt is dropped out in the salt filter. As the lye concentrates the gravity gradually rises to 28deg.-30deg. B., which is half crude glycerine and contains about 60 per cent. glycerine. Some operators carry the evaporation to this point and accumulate a quantity of half crude before going on to crude. After half crude is obtained the temperature on the evaporator increases, the vacuum increases and the pressure on the condensation drain goes up (using the same amount of live steam). As the liquor grows heavier the amount of evaporation is less, and less steam is required necessitating the regulation of the steam pressure on the drum. When a temperature of 210deg. F. on the evaporator, with 26 or more inches vacuum on the pump is arrived at, the crude stage has been reached and the liquor now contains about 80 per cent. glycerine in which shape it is usually sold by soap manufacturers. A greater concentration requires more intricate apparatus. After settling a day in the crude tank it is drummed. Crude glycerine (about 80 per cent. glycerol) free from salt is 33deg. B., or has a specific gravity of 1.3. A sample boiled in an open dish boils at a temperature of 155deg. C. or over. TWITCHELL PROCESS. The Twitchell process of saponification consists of causing an almost complete cleavage of fats and oils by the use of the Twitchell reagent or saponifier, a sulfo-aromatic compound. This is made by the action of concentrated sulfuric acid upon a solution of oleic acid or stearic acid in an aromatic hydrocarbon. From 0.5 per cent. to 3 per cent. of the reagent is added and saponification takes place from 12-48 hours by heating in a current of live steam. The reaction is usually accelerated by the presence of a few per cent. of free fatty acids as a starter. Recently the Twitchell double reagent has been introduced through which it is claimed that better colored fatty acids are obtained and the glycerine is free from ash. The advantages claimed for the Twitchell process as outlined by Joslin[12] are as follows: 1. All the glycerine is separated from the stock before entering the kettle, preventing loss of glycerine in the soap and removing glycerine from spent lye. 2. The liquors contain 15-20 per cent. glycerine whereas spent lyes contain but 3-5 per cent. necessitating less evaporation and consequently being more economical in steam, labor and time. 3. No salt is obtained in the liquors which makes the evaporation cheaper and removes the cause of corrosion of the evaporator; also saves the glycerine retained by the salt.