d hence the more effective the
removal of dirt and grease. This explanation holds true for the
removal of grease from any surface, whether of the body, clothing,
furniture, or dishes.

207. Washing Powders. Sometimes soap refuses to form a lather and
instead cakes and floats as a scum on the top of the water; this is
not the fault of the soap but of the 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 magnesium salts. When soap is put into
water containing one or both of these, it combines with the salts to
form sticky insoluble 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
satisfactory 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, something must be
done to counteract the hardness. The addition of soda, or sodium
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 washing 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 their alkali content, cut the
grease and themselves act as cleansers. In some cities where the water
is very hard, as in Columbus, Ohio, it