As part of the perspiration deposited upon our skins is in the form of a delicate oil, and as this oil may become mixed with dirt, or dust, and form a mixture not readily soluble in water, it is at times advisable to add to the water something that will dissolve oil.

The commonest thing used for this purpose is soap, which is a combination of an _alkali_--most commonly _soda_, though occasionally _potash_ (lye) is used in the soft soaps--with a fat or an oil.

The combination of the two, which we call soap, has been invented for two reasons; one, that it makes a convenient, solid form in which the alkali, needed to dissolve the body oil, can be used in such strength as not to burn or injure the skin; the other, that the fat in the soap will, to some extent, take the place of the natural oil, or fat, which it washes off.
Necessary as soap is, it should be used very moderately.

You should never lather and scrub your skin as if it were a kitchen floor, for the reason that, with the dirt, the alkali also washes and dissolves out a considerable amount of the natural oil of the skin, and leaves it harsh and dry.

On this account, it is best not to use soap upon the covered portions of the body, and in the full bath, oftener than once or twice a week; and upon the face, oftener than once or twice a day.