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.
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. But the hands may be washed with soap more frequently.
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.
Unless the dirt be of some infectious, or offensive, character, it is often best to content yourself with washing off just the “big dirt,” and wait for the bubbling up of the perspiration through your skin to bring the deeper dirt up to the surface, and wash that off later, in the course of two or three hours.