370 CHEMISTKY. 



true chemical salt. In the manufacture of soap, lime, so- 

 dium hydrate, or potassium hydrate may be used ; calcium, 

 sodium, or potassium stearate being formed. Natural fats, 

 however, are composed of stearin, palmitin, and olein, con- 

 sequently the soaps are mixtures of stearates, palmitates, 

 and oleates of the bases named, and not pure stearates. 

 If we consider the formation of only one of these bodies, 

 it can be expressed in a rather complex equation, thus : 



Caustic soda or Olive-oil or Gn ^- , i i rn 



Sodium hydrate. Glyceryl oleate. Sodmm oleate ' Glycerin. 



3(NaHO) + (C,H.)" / (C 18 H,,0 1 ), = 3(NaC 19 H 33 O 2 ) + (C 3 H 5 )"'(HO) 3 



518. Hard and Soft Soap. Hard soaps are formed by 

 soda, and soft by potash. A potash soap can be converted 

 into a soda soap by means of common salt. This is often 

 done by soap-makers on a large scale. The soft soap is 

 dissolved in boiling water, and salt is thrown into it. 

 There is a collection of soda soap on the surface of the wa- 

 ter, which on cooling becomes hard. The chemical change 

 is this : The chlorine of the salt goes to the potassium of 

 the soap, forming potassium chloride, while its sodium goes 

 to the stearic and oleic acids to form stearate and oleate 

 of sodium. 



The cleansing power of soap in washing depends chiefly 

 upon the fact that the water used with them sets free some 

 of the alkali from its combination with the fatty acids, so 

 that we have a mixture of caustic alkali and fat-salts, the 

 alkali by its union with the greasy matters in the cloth 

 cleansing it, and the salts the stearate and oleate by their 

 lubrication, keeping the cloth pliant, and thus making the 

 operation easy. The alkali would not answer alone, be- 

 cause it would by shrinking the fibre of the cloth render it 

 rigid, and thus prevent its perfect cleansing, and at the 

 same time would injure it .by its too great causticity. 



Alcohol dissolves soaps, and the common soap liniment 



