We must first consider what the substance is that has to be removed. The fabrics and upholstery used in furnishing catch dust which contains lint, grit, organic material from our bodies, and bacteria. Fabrics also become spotted (see next chapter). The walls and ceilings, floors and cracks, catch dust. All wood and glass surfaces become soiled from the touch of even clean fingers, and the moisture of the air mixed with dust dulls them. Metal surfaces oxidize, and this oxidized layer must be rubbed off.