The great enemy of iron when used in architectural or engineering work is oxygen. We would not be understood to ignore other causes, the tendency of which is to hasten the destruction of bridges, etc., such as expansion, the production of a crystalline state of the metal by vibrations, etc., but these causes apply only to special cases, while in all cases, unless something interferes, oxygen slowly but surely gnaws away at every bit of iron exposed to its action. A great many methods have been employed to prevent the rusting of iron, by which is meant its chemical union with the oxygen of air, water, or other medium in which it may be placed. In the case o'1 saline waters, the reactions are more complicated, but the final result is the same, namely, the oxidation and disintegration of the metal. Unprotected iron rusts away much faster in such waters than in common air ; but exposed to the action of the ordinary substances, to be found in all places where structures o. iron are located, the ultimate destruction of such structures is merely a question of time. But while the vibratory motion of iron tends to render it brittle, and change its physical character from a fibrous to a crystalline material, such motion acts, in some yet unexplained manner, to combat the affinity of oxygen for iron. Hence the old proverb that “ the used key is always bright,” has more foundation than the polishing effect of wear. In machinery it is common to paint or otherwise protect the stationary parts, while the moving parts have been found not to require much protection, when properly shielded from damp. In many cases castings will stand in a shop just as they have been taken from the sand, without rusting, being protected by a thin film of silica from the melting of the sand during the process of pouring, but as soon as exposed to the action of water this protection fails, and they rapidly become coated with rust. The processes most generally applied to shield iron from the action of oxygen, have for their object the isolation of the metal from this gas. The coating of iron with metals is one of the most important of the means employed for this purpose; tin and zinc being the metals most frequently used. But these metals will not permanently protect iron in all situations, and they cannot in many instances be applied. Another class of substances are paints, tar, linseed oil, etc., which form coatings upon the surface of iron and thus isolate it from oxygen. None of these can, however, be relied upon as a permanent protection ; and they have to be from time to time renewed, upon parts where the metallic surface has become exposed. It has been regarded by some as quite doubtful whether any cheap and practicable method for the prevention of iron rust, that will permanently secure this object can be devised, yet it would seem, with all the great resources of modern chemistry, this problem should be capable of solution. As yet iron cannot compete with stone in structures designed to endure the effect of time, without repeated attention to keeping its surface covered with some protective covering ; and until it is enabled to do this by some improvement in methods of protection, its use for engineering and architectural purposes can never entirely supersede that of stone, if, indeed, it can ever compete with stone in other respects. What is wanted is somethingapplicable to large or small pieces of iron, and which will answer to ward off the attacks not only of the common atmospheric oxygen, but will also remain unaffected by acids or salt waters. Who will give this to the world ?