One patent, by a complicated process, effects a union of metallic zinc and iron; this, granulated and ground fine, then mixed with red lead and oil, makes the paint. It is said to be the best of all the patented stuffs. It is astonishing how many use oils derived from coal, peat, or resin, and tars of the same. There are about fifty patents for this object and with all of them before their eyes, the British Society for the Advancement of Art still hold the $5,000 reward for a pigment or covering which will perfectly protect from rust and fouling. However they may puff their products for selling, no one has the temerity to claim that they deserve the reward. We think it would be difficult to find so many expedients ever before adopted for the accomplishment of any one object. These are all English patents, England having necessarily been obliged to use iron for vessels from its cheapness as well as its consequent first introduction there. In the United States no patents worth mentioning have been granted. The first requisites for a pigment or coating for iron are, that it should not contain any copper--the corrosive action of that metal on iron being intense. Then if for work exposed to air it should form such a coating as to be impervious to that gaseous fluid, and be so constituted chemically as not to be oxidizable by it; if under water--especially sea water--to be impermeable to moisture, so elastic as not to crack, so insoluble as not to chloridize; to form a perfect, apparently hard, coating: and yet wear just enough to keep off incrustation, barnacles, or growth of grass. In fact, this slow wearing away is the only preventive of fouling in iron vessels. Wooden bottoms may be poisoned by solutions of copper--and that metal has no superior for such uses, especially when it is combined in mixture with mineral or resinous tars and spirits--these compounds, however, are not only useless on iron bottoms, but also injurious. What then is _the_ substance: 1st. One of the oxides of lead (red lead). 2d. The purest oxide of iron to be found. If properly made these articles can be carried to no higher state of oxidation, and respectively, as to order named, they have no superiors for body and durability. By preference, 1st, red lead, either out of or under water; 2d, Prince's oxide of iron only, out of water. The color of these paints--the first red, the latter brown, may be hidden by a coat of white or tinted color. If there were to be had in combination as a white paint, an oxide of lead and an oxide of zinc, it would be immensely superior to either, but that such has not been produced is rather the fault of carelessness than of possibility. Zinc protects iron with great effect, but it is too rapidly worn in the effort to be of lasting value. Hence the great desideratum, the yet to be, the coming pigment is a white oxide of lead or a combined white oxide of lead and white oxide of zinc, without sulphates or chlorides.