178 GENERAL SCIENCE 



substance in the soap known as an alkali, the amount of 

 which if excessive may act destructively upon the skin and 

 possibly upon the fibre of the cloth. The soapy compound 

 made with the grease should be removed by thorough rinsing. 



Water used for cleaning purposes is said to be "hard" 

 where it has in solution compounds of calcium and of magne- 

 sium commonly spoken of as "lime." A chemical change oc- 

 curs between these compounds and the soap so long as the sup- 

 ply of them in the water lasts, preventing any cleansing action 

 by the soap. One product of such reaction (chemicaFchange) 

 between the soap and the lime in the water is the insoluble 

 dirty scum that is such an annoyance in the use of hard 

 waters. This "lime soap" becomes lodged within the fibre 

 of clothing washed in hard water, injuring the fabric and 

 giving the clothing a dirty appearance. The hardness of 

 water may be measured by its soap-consuming power. 



The waste of soap thus indicated has led to a general use of 

 washing powders. These essentially are strongly alkaline 

 substances in powdered form that dissolve quite readily in 

 warm water. As a rule they consist very largely of some 

 substance such as sodium carbonate (Na 2 CO 3 ), commonly 

 known as "sal-soda" or "washing soda", which maybe 

 purchased in bulk at a few cents a pound. Great care must 

 be exercised in the use of all washing-powders to secure com- 

 plete solution of them before their use with clothing, and to be 

 certain that the solution is not so strong as to injure the fabric. 



A laundry bluing in common use is a compound of iron, and 

 garments must be thoroughly rinsed to free them of all alkali 

 before using it. Otherwise there will be left on the clothing 

 spots of iron rust (oxide of iron) as the result of chemical 

 changes in the bluing due to the action of the alkali, or blue 

 streaks may appear where soap was left in the cloth. There 

 should be bluing enough left in the cloth to destroy the slight 

 natural yellow of cotton and linen fibre. 



