and be of such fluid nature as to flow together after the brush so that the resulting covering, or skin, is one of even thickness.

II. Chemical Properties.   That it must not only dry, or oxidize, fast enough, but dry simultaneously throughout; not harden on the surface and remain soft underneath, or in painters' parlance, "skin over."

III. Physical Properties.

(1) Hardness. That it must be of such nature that when it has formed a skin upon the surface of the metal it can take other coats of paint without softening under them. And further, that the skin formed must be hard enough to wash clean by rain-fall.

(2) Indestructibility. That it must wear well, provided a sufficient number of coatings of it is applied.

(3) Impenetrability. That it must ex-