ACTION OP WATER ON IRON. 277 



dry. On being wet, they expand^ and force themselves up so 

 close to the interior of the pipe, and at their joints, that the 

 whole cast-iron pipe becomes staunchly lined with a casing of 

 wood, which cuts off all communication with the corroding 

 agents. 



60. The diffei'ence in the rate of action of air and water, but 

 still more of acids, on different specimens of cast iron, contain- 

 ing variable minute quantities of foreign matter, is very remark- 

 able. The iron obtained by the remelting of old coal-gas 

 retorts is of a quality closely approaching what is called " Re- 

 finery Pig, or No. 4 '," in fact, by a little management, it may 

 be forged at once on the anvil into a bar. It is found to contain 

 a large quantity of sulphur, and an unusual amount of silex, in 

 some cases as much as 18 per cent., with very little carbon. 



A fragment of this iron placed in hydrochloric acid, diluted 

 with 15 volumes of water, will not be dissolved for months, a 

 coat of silex being apparently formed ; while a fragment of the 

 iron before mentioned as containing chloride of sodium in mi- 

 nute quantity, with a large proportion of sulphur and carbon, 

 will dissolve in the same acid almost as readily as sugar in 

 water. 



Sulphur, also, which acts on wrought iron with such intensity 

 at a bright red heat, acts on black cast iron, according to 

 Colonel Evans, at the same temperature with comparative 

 feebleness.— r^y^nn. de Chim. vol. xxv. p. 107.) 



61. Both these highly sulphuretted irons seem to coordinate 

 with the highly carburetted ones, or black cast irons, the sulphur 

 appearing to form a definite compound, intermixed with the rest 

 of the substance mechanically. 



62. Of the relations in respect of corrodibility subsisting be- 

 tween the different known varieties or " makes" of bar iron and 

 of steel, but very little is established with any certainty. The 

 hardest kinds of malleable iron generally appear to oxidize 

 slowest, but this is not universally true, as Swedish iron is acted 

 on by air and water with great rapidity. 



63. Karsten states that cold short iron is rapidly corroded. 

 Steel appears less corrodible than any variety of wrought iron ; 

 but of none of these is there any precise knowledge on our sub- 

 ject, or that approaches numerical results, which alone are of 

 practical use in directing the engineer, or indeed the jihysicien. 

 Neither is our actual knowledge more advanced as to the variable 

 effects of corrosive action on the same iron, of different waters, 

 su9h as are commonly met with, containing their usual mineral 

 ingredients in solution. We know not whether foul water, or 



VOL. VII. 1838. T 



