SOAP. 49 



stearin (a solid) and olein (a liquid). When making this soap, 

 the tallow is dissolved in a weak solution of soda ; the result 

 being that soap and glycerine are produced. The soap is 

 separated from the water by an addition of common salt, which 

 is capable of performing this office, because soap is insoluble in 

 a solution of salt. The soap is then removed from the " ley " 

 and dried. Soft soap is made by dissolving a liquid fat (for 

 instance, fish oil, which is largely composed of olein) in a solu- 

 tion of potash ; the resulting products being potassium oleate, 

 4 to 5 per cent, of glycerine, and probably some free potash or 

 some free oil. Economy being a matter of great importance 

 in such an industry, it is natural to suppose that there would be 

 nearly always an excess of free potash. As the addition of 

 common salt to a solution of potassium oleate would cause the 

 formation of ordinary hard (soda) soap and potassium chlo- 

 ride, salt cannot be used here for purposes of separation, and 

 consequently the soft soap is obtained by evaporating the 

 solution. Hence, soft soap contains glycerine and all the im- 

 purities that were in the pan. Yellow soap differs from ordi- 

 nary white soap, by the fact of its possessing a considerable 

 percentage of resin, which, by rendering it harder, and by de- 

 creasing its solubility in water, makes it more economical to 

 use. Glycerine soap is a mixture of about equal parts of soap 

 and glycerine. 



The darkening effect which the continued application of soap 

 has on leather, appears to be chiefly due to the action which the 

 free alkali of the soap has on the tannic acid in the leather. 

 The addition of either soda or potash to a solution of tannic 

 acid turns it brown ; the colour becoming deeper from exposure 

 to the air. As soft soap contains more free alkali than ordi- 

 nary white or yellow soap, its darkening effect on leather will 

 be more marked. 



The solution of soap in water causes its decomposition 

 (hydrolysis) into its fatty acids and its alkali. " Its cleansing 



4 



