ACTION OF WATER ON IRON. 259 



20. If malleable iron or steel have been snbjected to the sol- 

 vency of sea water, carbon and sometimes silicon are deposited 

 in small quantities ; but when cast iron is acted on more re- 

 markable results follow. After it has remained for a length of 

 time immersed, the metal is found wholly removed, and in its 

 place a pseudomorph of its original size remains, as first observed 

 by Priestley, consisting of a carbonaceous substance, analogous 

 to plumbago, mixed with oxides of iron, and which frequently, 

 but not invariably, possesses the property of heating or in- 

 flaming spontaneously when exposed to air. There have been 

 unfortunately, as yet, but few cases of this remarkable change, 

 which requires the lapse of time to take place, carefully ob- 

 served ; and it is as yet by no means clear how it is produced, 

 what is its precise composition, or to what is owing the rise in 

 its temperature on exposure to air. 



21. It is remarkable, that not cast iron alone is subject to 

 this change ; under circumstances but little understood as yet, 

 the purest malleable iron is alike converted into what we shall 

 for brevity call phnnhago. 



Karsten mentions, that when iron, whether wrought or cast, 

 has been long exposed to water holding in solution alkaline or 

 earthy salts, it is at length dissolved ; that when hard bar iron 

 had remained some centuries in sea water it was altogether dis- 

 solved, and a mass of carbonaceous matter remained, as though 

 it had been submitted to the prolonged action of a diluted acid. 

 This change, he says, is generally attributed to the decomposi- 

 tion of the carbonic acid contained in the sea water; but it is 

 much more likely that, in the long run, the sulphates and chlo- 

 rides contained in the sea water are decomposed likevdse by 

 the iron. 



The writer possesses a portion of an ancient anchor taken up 

 in the port of Liverpool, the iron that remained of which was 

 of remarkable purity, and which was converted into plumbago 

 of unusual hardness and brilliancy to the depth of half an inch. 

 This plumbago did not heat on exposure. Its specific gravity is 

 1'773- This fact militates against anobservation made by Hatchet, 

 and repeated by Becquerel, that anchors and other objects of 

 forged iron sustain no alteration in sea water but oxidation, 

 from which we must suppose that the contact of iron and plum- 

 bago in the cast iron produces a voltaic current, which accele- 

 rates the action of the latter. 



Berzelius' opinion is, that the carbonic acid contained in the 

 water dissolves and removes the iron. He quotes an instance 

 of the guns of a vessel which had foundered off Carlscrona, 

 which, when taken up fifty years afterwards, were found nearly 



