EFFECT OF SEA-WATER ON IRON. 377 



have lost 3^7 part of its weight. The wrought iron had only 

 lost - 6 s l f o f ^ s weight, being in the proportion of two lost by 

 the cast iron to one only lost by the wrought iron, while the 

 bronze cube had only lost T^QO"O ^ l *s weight, which is decidedly 

 in favour of the bronze in the ratio of three to one. 



The cast- and wrought-iron cubes being accurately weighed, 

 were again plunged into a strong solution of 1 measure of 

 muriatic acid to 25 measures of Thames water, when, after 

 remaining twenty-one hours, the cast-iron cube was found to 

 have lost ^ of its weight, and the wrought iron only ^-Jg of its 

 weight, being in the proportion of 8 to 1 in favour of wrought 

 iron. 



The important experiments of the late Mr. K. Mallet, 1 

 M.I.C.E., on specimens sunk in the sea, showed that the amount 

 of corrosion decreased with the thickness of the metal, and that 

 from -^Q to -^Q of an inch in castings 1 inch thick, and about f$ 

 of an inch of wrought iron will be destroyed in a century in 

 clear salt water ; this is equal to 1*5 to 1 in favour of cast iron. 



Stevenson records instances of far more rapid deterioration. 2 



Experiments made at Dublin, by the late Mr. R Mallet, 

 M.I.C.E., show that cast iron, freely exposed to the weather 

 and to all its atmospheric precipitations, was corroded nearly 

 as fast as if in clear sea-water, the specimens being wholly 

 unprotected in both cases. 8 



On the whole, it may be taken as a fact that cast iron is 

 less liable to corrosion, when immersed in sea- water, than 

 wrought iron ; this may be due to the surface of the cast iron 

 being covered with a skin of silicate of protoxide of iron pro- 

 duced by the molten metal fusing the sand upon the surface 

 of the mould. 



Iron exposed to tidal wash and alternately wet and dry 

 is more liable to rapid waste, unless well cared for and pro- 

 tected, than when wholly immersed in water, or wholly exposed 

 to the air. 4 



Cast iron, when exposed to the action of sea- water, some- 

 times slowly decomposes and becomes quite soft. The iron 

 apparently dissolves, leaving behind a graphite or plumbago. 

 This action is, however, very superficial and very slow, and is 

 very far from being general. This softening process is not 



1 " Stevenson on Harbours," 2nd ed. p. 187. a M.P.I.C.E., pp. 188, 189. 



Ibid., vol. ii. p. 175. * Ibid., vol. iv. p. 329. 



