CLEANSING PROPERTIES OP SOAP. 271 



substances come in contact with an alkali, in solution at 

 an elevated temperature, they undergo an entire change, 

 and on this change the whole process of soap making 

 depends. 



986. The soap made in the farm-house is that known 

 as soft soap, and is formed by the union of potash with 

 more or less fatty matter. Hard soaps are made by the 

 use of soda, with which potash is sometimes mixed. 

 Potash will not harden when water is present, as it 

 always is in considerable quantities in soft soap. But 

 soap made with soda will absorb more than its own weight 

 of water without losing its consistency. 



987. The soft soaps are generally made of soft fats, 

 while the hard soaps are more frequently made from 

 tallow. In making castile soap, olive oil and soda are 

 used, and its peculiar marbled appearance is produced by 

 the mixture of iron rust. Rosin is very often added in 

 the manufacture of common or yellow soaps. 



988. Rosin soaps dissolve or form lather so readily, 

 that they are generally believed to be very effective, but 

 they are by no means so economical as the soda soaps, 

 their cleansing properties being inferior. 



989. The cleansing properties of soap depend mainly 

 on its alkaline ingredients. When brought in contact 

 with the impurities of clothing, or of the skin, which are 

 made up of a greater or less quantity of oily matter 

 derived from the exhalations of the body, together with 

 dust and other foreign substances, the alkali of the soap 

 readily seizes hold of the oily matters and dissolves or 

 removes them. 



990. If water is used without soap, it often fails to 

 cleanse thoroughly, as it has no affinity for oily 

 substances, and therefore leaves them and whatever has 



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