158 CHEMISTRY FOR AGRICULTURAL STUDENTS 



a cylinder, and add to it dilute sulphuric acid till slightly acid 

 Note the separation of solid stearic acid. 



NaCisHg^Og + H2SO4 = NaHS04 + YLC^^.^O^, 



For preparing glycerine and the fatty acids on a large scale, fats are hydro- 

 lysed by distilling with superheated steam. The distillate consists of a 

 solution of glycerine and a separate layer of the fatty acid. The latter is 

 employed in the manufacture of "stearine" candles; the pure glycerine is 

 obtained by evaporating the aqueous liquid. Fats are sometimes described 

 as glycerides of the fatty acids. Thus stearin is the glyceride of stearic 

 acid, olein the glyceride of oleic acid, etc. 



It is evident that soaps are oleates, palmitates, or stearates of 

 potassium or sodium. Potassium soaps are soft ; sodium soaps 

 are hard. When dissolved in water for washing purposes, a 

 little hydrolysis occurs, and soda or potash is set free, which 

 dissolves the grease from, and therefore cleanses, the hands. 

 When used with hard water, a curd is formed owing to the for- 

 mation of insoluble oleates, palmitates, and stearates of calcium 

 and magnesium, the soaps of the alkali metals being the only 

 soaps soluble in water. To the solution of the soap add dilute 

 solutions of calcium and magnesium salts. Note the curdy pre- 

 cipitates of the calcium and magnesium salts of the fatty 

 acids. 



2NaCi8H3502 + CaH22C03 = 2NaHC03 + Ca2Ci8H3502 

 2NaCisH3502 + MgSO^ = Na.SO, + Mg2Ci8H3,02. 



By making use of these reactions, the total, permanent and 

 temporary hardness of tap water may be compared and approxi- 

 mately determined. To determine the total hardness place 70 

 c.c. of the water in a stoppered bottle, and run in — in very small 

 quantities at a time, until a permanent lather is produced after 

 shaking — a standardised solution of soap in weak alcohol (sup- 

 phed), of such a strength that i c.c. of the solution will exactly 

 precipitate .001 gram of calcium carbonate in a state of solu- 

 tion in 70 c.c. of water. Each c.c. of the soap solution used 

 will then represent one " degree of hardness," i,e. one grain of 



