144 HOUSEHOLD CHEMICALS 



Commercial soaps are made from a great variety of sub- 

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

 oil ; very cheap soaps are made from rosin, cottonseed 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 New York, Columbus, and some other 

 cities garbage is used as a source of revenue ; the grease from 

 the garbage being sold for soap making, and the tankage for 

 fertilizer. 



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 into the water. Soap affects grease and water very much 

 as the white of an egg affects oil and water. Oil and water do 

 not mix, the oil remaining separate on the surface of the water. 

 But if a small quantity of white of egg is added to oil and 

 water an emulsion is formed and the oil separates 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 emulsion, and the more effective the re- 

 moval of dirt and grease. This explanation holds true for the 

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

 clothing, furniture, or dishes. 



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 water. As water 

 seeps through the soil or flows over the land, it absorbs sub- 

 stances which modify its character and which, in some cases, 

 render it almost useless for household purposes. Most of us 

 are familiar with the rain barrel of the country house, and know 



