356 Proceedings of the British Association. 



tuent carbon. The present Report, too, extends the inquiry to 

 wrought iron and steel, of which between thirty and forty varie- 

 ties have been submitted to experiment. The results show that 

 the rate of corrosion of wrought iron is in general much more 

 rapid than that of cast iron or of steel. The finer the wrought 

 iron is, and the more perfectly uniform in texture, the slower 

 and more uniform is its corrosion. Steel corrodes in general 

 more slowly, and much more uniformly, than wrought or cast 

 iron. The results of the action of air and water in the several 

 classes of iron have been examined and chemically determined. 

 The substance spoken of as plumbago was next described. It is 

 produced by the action of air and water on cast steel, especially 

 that in the raw ingot, in the same way as it is in the case of cast 

 iron. A quantity of plumbago, found in the wreck of the Royal 

 George, absorbed oxygen on exposure to the air with such ra- 

 pidity, that it became nearly red hot. Mr. Mallet next described 

 a method of protecting iron by a modification of the zinc pro- 

 cess. It was found impossible to cover the surface of iron with 

 zinc, to which it had no affinity. The first process was to clean 

 the surface of the iron, taking off the coat of oxide, and then im- 

 mersing it in double chloride of zinc and ammonium, which cov- 

 ered it with a thin film of hydrogen, by which its affinity for the 

 zinc is much increased. The iron was then covered with a triple 

 alloy of zinc, sodium, and mercury. Mr. Mallet produced sev- 

 eral specimens of his alloy, one of a bolt to be driven into a ship's 

 side, and another a cannon shot covered with his preparation, and 

 exposed to the weather on the roof of a building, and which was 

 perfectly preserved. Cannon balls were so much oxidized by 

 exposure to atmospheric influences, that in five or six years they 

 became useless. The French Institute had been engaged in ex- 

 periments to protect these, and had tried zinc, but had been com- 

 pelled to abandon it. Mr. Mallet also brought under the notice 

 of the Section a method of preventing the fouling which takes 

 place on the bottoms of iron ships, especially in tropical climates, 

 by means of which invention he had ascertained that plants and 



bottom. Ac- 



Mr 



ed by the trains passing over the rails always in one direction, 

 and takes place when they pass in both directions. Mr. M. had 

 made some experiments in order to determine this point, which 



