WASHING POWDERS 22$ 



water. As water seeps through the soil or flows over the land, 

 it absorbs and retains various soil constituents which modify 

 its character and, 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 that the housewife 

 prefers rain water for laundry and general work. Rain 

 water, coming as it does from the clouds, is free from the 

 chemicals gathered by ground water, and is hence practically 

 pure. While foreign substances do not necessarily injure 

 water for drinking purposes (Section 69), they are often of such 

 a nature as to prevent soap from forming an emulsion, and 

 hence from doing its work. Under such circumstances the 

 water is said to be hard, and soap used with it is wasted. 

 Even if water is only moderately hard, much soap is lost 

 The substances which make water hard are calcium and mag--' 

 nesium salts. When soap is put into water containing one or 

 both of these, it combines with the salts to form sticky insolu- 

 ble scum. It is therefore not free to form an emulsion and to 

 remove grease. As a cleansing agent it is valueless. The 

 average city supply contains so little hardness that it is satis- 

 factory for toilet purposes ; but in the laundry, where there is 

 need for the full effect of the soap, and where the slightest 

 loss would aggregate a great deal in the course of time, some- 

 thing must be done to counteract the hardness. The addition 

 of soda, or calcium carbonate to the water will usually produce 

 the desired effect.. Washing soda combines with calcium and 

 magnesium and prevents them from uniting with soap. The 

 soap is thus free to form an emulsion, just as in ordinary 

 Water. Washing powders are sometimes used instead of wash- 

 ing soda. Most washing powders contain, in addition to a 

 softening agent, some alkali, and hence a double good is 

 obtained from their use ; they not only soften the water and 

 allow the soap to form an emulsion, but they also, through 



CL. GEN. SCI. 15 



