oil to use up the gallon as it will of the paint to use it up. From this we conclude that a layer of the paint is about three times as thick as a layer of the oil. Experiments of this kind demonstrate that one of the functions of pigment is to increase the thickness of the layer of dried paint, and that this increase of thickness is in direct proportion to the volume or fineness of the pigment. They also determine that, given the same volume of oil and the same weight of pigment, the greater the volume of the latter—that is, the finer the division of its particles—the more slowly will the paint dry and the longer will it wear.

Pigments in general use for structural oil paints, as related to their mutual action with the steel which they cover, may be classified as (1) stimulators of rusting, or pigments whose   moist   contact   with  steel   pro-