It must bo remembered that thin plates and small pieces of iron decay very rapidly, particularly in damp situations; therefore they should be well secured against rust by being painted as soon as they are made. Smeaton, writing on this subject, says, "I had observed that when iron once gets rust, so far as to form a scale, whatever coat of paint or varnish is put over this, the rust will go on progressively under the paint." The method he used to prevent iron from rusting was to heat it to about a blue heat, and immediately strike it over the surface with raw linseed oil: the next day, if properly done, it appears as if a coat of varnish had been laid on.* By this method the pores of the iron become filled, and effectually protected from corrosion.

Coating the iron while hot with a mixture of tar and asphalte is a good preservative.

Another method, that is easily applied to small articles, consists in heating the metal, and rubbing it over, while hot, with wax. By this process the iron acquires an extremely uniform coating.

Nails and other small fastenings might be rendered much more lasting by boiling them in linseed oil. This is often practised by slaters to protect their nails from rust.

As it is difficult to heat large articles, a coating that can be applied in a cold state is much better. One that dries quickly, and, it is said, perfectly preserves from rust the metals upon which it is laid, is described in the ' Repertory of

* ' Historical Account of the Construction of Eddystone Lighthouse,' p. 182.

Arts.' * It is prepared as follows: Grind to an impalpable powder 1 part (by weight) of black-lead (plumbago), with which mix 4 parts of sulphate of lead, and 1 part of sulphate of zinc: to this mixture add, by degrees, 16 parts of boiled linseed oil.

Coating with zinc, called galvanizing, is now commonly used, but it is destroyed by the fumes of sulphuric and muriatic acid, consequently it is more liable to decay where much coal is burned, or near the sea, than elsewhere.