200 ACIDS, ALKALIES, AND CLEANING 



tion of potash that was formed trickled out into little troughs in the 

 platform. The solution was collected in kettles, and boiled down to 

 form the home-made lye. A kettle of fat was melted over an open fire, 

 the lye was added to this, and the two were cooked together, often for 

 two or three days. The resulting " soft soap " was put away in barrels 

 for the next year's use. 



225. Action of Soap. When a sodium soap is dis- 

 solved, it is partly broken up, by the action of the water, 

 into sodium hydroxide and the fatty acids (cf. 218). 

 The sodium hydroxide acts upon the fats and oils of the 

 skin and clothing, and partly converts them into soaps, 

 so that they also are dissolved by the water. In using 

 soap we are using lye in a most convenient form, for its 

 caustic properties are so altered that it can cleanse fabrics 

 and the skin without injuring them. Toilet soaps are 

 " neutral to litmus/' because an excess of fat is used, so 

 that the amount of free alkali is small. Laundry soaps 

 have more free alkali. 



Soap has not only this chemical action, but the lather, 

 or suds, acts mechanically, entangling the undissolved fat, 

 dead skin, and dirt, and removing them. 



226. Soap and Hard Water. We have already learned 

 (cf. 82) that the hardness of water is its soap-consuming 

 power. Hardness is due to dissolved salts, especially 

 calcium carbonate (limestone) and calcium sulphate 

 (gypsum). When soap is put into a hard water, it acts 

 with these salts, forming the calcium salts of the fatty 

 acids. These are insoluble, and separate as a scum, called 

 lime soap. If soap is used in sufficient amount, it will 

 soften such a water; but soap is too expensive. Besides, 



