Most acids and bases will eat and corrode and discolor, while neutral substances will not; it is for this reason that soap, a slightly alkaline substance, is the safest cleansing agent for laundry, bath, and general work.  Good soaps, being carefully made, are so nearly neutral that they will not fade the color out of clothing; the cheap soaps are less carefully prepared and are apt to have a strong excess of the base ingredient; such soaps are not safe for delicate work.

205.  Soap.  If we gather together scrapings of lard, butter, bits of tallow from burned-out candles, scraps of waste fat, or any other sort of grease, and pour a strong solution of lye over the mass, a soft soapy substance is formed.  In colonial times, every family made its own supply of soap, utilizing, for that purpose, household scraps often regarded by the housekeeper of to-day as worthless.  Grease and fat were boiled with water and hardwood ashes, which are rich in lye, and from the mixture came the soft soap used by our ancestors.  In practice, the wood ashes were boiled in water, which was then strained off, and the resulting filtrate, or lye, was mixed with the fats for soap making.

Most fats contain a substance of an acid nature, and are decomposed by the action of bases such as caustic soda and caustic potash.  The acid component of the grease partially neutralizes the base, and a new substance is formed, namely, soap.

With the advance of civilization the labor of soap making passed from the home to the factory, very much as bread making has done in our own day.  Different varieties of soaps appeared, of which the hard soap was the most popular, owing to the ease with which it could be transported.  Within the last few years liquid soaps have come into favor, especially in schools, railroad stations, and other public places, where a cake of soap would be handled by many persons.  By means of a simple device (Fig. 157), the soap escapes from a receptacle when needed.  The mass of the soap does not come in contact with the skin, and hence the spread of contagious skin diseases is lessened.

[Illustration:  FIG. 157.—­Liquid soap container.]

Commercial soaps are made from a great variety of substances, such as tallow, lard, castor oil, coconut oil, olive oil, etc.; or in cheaper soaps, from rosin, cottonseed oil, and waste grease.  The fats which go to waste in our garbage could be made a source of income, not only to the housewife, but to the city.  In Columbus, Ohio, garbage is used as a source of revenue; the grease from the garbage being sold for soap making, and the tankage (Section 188) for fertilizer.