224 MAN^S CONQUEST OF SUBSTANCES 



simple device (Fig. 157), the soap escapes from a receptacle 

 when needed. The mass of the soap does not come in con- 

 tact with the skin, and hence the spread of contagious skin 

 diseases is lessened. 



Commercial soaps are made from a great variety of sub- 

 stances, such as tallow, lard, castor oil, coconut oil, olive oil, 

 etc.; or in cheaper soaps, from rosin, co.ttonseed oil, and 

 waste grease. The fats which go to waste in our garbage 

 could be made a source of income, not only to the housewife, 

 but to the city. In Columbus, Ohio, garbage is used as a 

 source of revenue ; the grease from the garbage being sold 

 for soap making, and the tankage (Section 188) for fertilizer. 



206. Why Soap Cleans. The natural oil of the skin 

 catches and retains dust and dirt, and makes a greasy film 

 over the body. This cannot be removed by water alone, but 

 if soap is used and a generous lather is applied to the skin, 

 the dirt is "cut" and passes from the body into the water. 

 Soap affects a grease film and water very much as the white of 

 an egg affects oil and water. These two liquids alone do not 

 mix, the oil remaining separate on the surface of the water ; 

 but if a small quantity of white of egg .is added, an emulsion is 

 formed, the oil separating into minute droplets which spread 

 through the water. In the same way, soap acts on a grease 

 film, separating it into minute droplets which leave the skin and 

 spread through the water, carrying with them the dust and dirt 



\ particles. The warmer the water, the better will be the emul- 



1 sion, and hence the more effective the removal of dirt and 



grease. This explanation holds true for the removal of 



grease from any surface, whether of the body, clothing, 



furniture, or dishes. 



207. Washing Powders. Sometimes soap refuses to form 

 a lather and instead cakes and floats as a scum on the top 

 of the water; this is not the fault of the soap but of the 



