WHAT USES DO WE MAKE OF WATER? 91 



separates. You have proved that water and oil under 

 ordinary conditions do not mix. Now start again, but put 

 a few shavings of soap in the water. Shake to make 

 a soapy solution. Add the oil and shake vigorously 

 as before. This time the oil does not separate from the 

 water. Shaking divides the oil into many exceedingly 

 fine droplets. 'In water alone they quickly combine and 

 separate out from the water, but when coated with soap 

 which is in the water, they keep their finely divided state 

 and remain mixed with the water for a very long time. 

 Thus you have shown that when soap is present to lend 

 its aid, water and oil will mix. This mixture is different 

 from solution and is called an emulsion. A kerosene 

 emulsion is an insect spray used to kill aphids. Milk 

 is an emulsion. It has oil in the form of butter fat dis- 

 tributed in very fine particles. These rise very slowly, 

 and when they form a layer on top of the milk, they are 

 known as cream. 



Value of Soap in Cleaning. It is largely the grease and 

 oils that hold the dirt particles to the hands and clothing. 

 Water alone has little cleaning value because it cannot 

 remove the oil. But when soap is added to the water 

 it forms an emulsion with it and this loosens the dirt. 

 For this reason soap is a valuable cleaning aid. When one 

 lives where the water is hard, containing minerals like 

 calcium compounds in the water, the soap is destroyed. 

 Such water must be softened sometimes by boiling, some- 

 times by adding washing powder or other chemicals 

 before using soap. Otherwise a great deal of soap will be 

 wasted. 



Why We Need Water. Do you know that your body 

 is over 65 per cent water, and that some animals such 

 as the jellyfish are 99 per cent water? Have you thought 

 that the plants we eat, stems like celery, roots like 

 radishes, all contain a large per cent of water, and that 



