SOAPS. 421 



diluted oil of vitriol. The material floats on the dilute 

 acid, gradually losing lime, and becoming transparent 

 by its action. Sulphate of lime or gypsum is formed 

 at the same time and sinks to the bottom of the vessel. 

 The stearic and oleic acids are drawn off while yet 

 warm, and run into cubical moulds. The latter is sub- 

 sequently removed from the mixture by gentle heat 

 and pressure. The remaining stearic acid is then re- 

 melted and allowed to cool slowly. It is thus ob- 

 tained in a brilliant white mass, of crystalline texture, 

 with the lustre of mother of pearl. This material is 

 principally employed in the manufacture of candles. 

 Its superiority to stearine for this purpose, consists in 

 the fact that it is less softened by heat. The two sub- 

 stances differ in their melting point about ten degrees. 

 1048. SOAPS. Soaps are compounds of 



How are pot- . ' . 



ash and soda stearic and oleic acids with caustic potash 



~ or soda '* The y are P roduced b y boiling 

 fats with either of the alkalies, till the 

 mixture becomes nearly or quite transparent. The 

 glycerine which is expelled from the fats in the process, 

 remains mixed with the soap which is produced. Pot- 

 ash soaps are soft. Soda soaps may be converted into 

 a floating coagulum, and separated from the water used 

 in their preparation by means of common salt. This 

 method is employed to give them their hardness. The 

 action depends on the insolubility of the soap in salt 

 water. Salt added to potash soap seems to have the 



* In the ordinary preparation for soap making, the lye is made to 

 pass through lime in the leach tub, that its carbonic acid may be par- 

 tially removed. 



