Solution and Chemical Action 357 



FIG. 184. The lye has changed the wool cloth to a jelly. 



How soap is made. When lye and grease are boiled 

 together, they form soap. You cannot very well make 

 soap in the laboratory now, as the measurements must 

 be exact and you need a good deal of strong lye to make 

 it in a quantity large enough to use. But the fact that 

 soap is made with oil, fat, or grease boiled with lye, or 

 caustic soda, which is almost the same thing, shows 

 why a soap must be 99ro% pure, or something like 

 that, if it is not to injure " the most delicate fabric." 

 If a little too much lye is used there will be free alkali 

 in the soap, and it will make your hands harsh and sore 

 and spoil the clothes you are washing. A " pure " soap 

 is one with no free alkali in it. A " strong " soap is one 

 that does have some free alkali in it; there is a little 

 too much lye for the oil or fat, so some lye is left un- 



