642 TRANSACTIONS OF THE 



This leaking, if not excessive, is frequently cured by the for- 

 mation of a sufficient coating of oxide upon the sides of the 

 crystals, composing the spongy mass, to fill the interstices and 

 prevent the further passage of the water. But at the same time 

 that this process cures the leaking, it increases the original weak- 

 ness, and shows that the work of destruction is going on, and 

 that the destructive agent occupies the whole thickness of the 

 metal, instead of being confined in its action to the outer surface 

 alone. And this internal oxidation proceeds, in some cases, to 

 such an ey:tent, that parts of pipes may be broken almost like 

 pieces of pottery ware, and granulated by being pounded in a 

 mortar. Iron of this quality is therefore unsuitable for water 

 pipes, especially where the head is sufficient to subject them to 

 any considerable pressure, as it possesses neither the requisite 

 strength nor durability. 



If on the other hand, they are east from No. 3, or white iron, 

 they will also lack strength. Iron of this quality, in the first 

 place, being weak; and in the next, castings made from it are 

 liable to contract very irregularly in cooling, by which some parts 

 are frequently brought Into such a state of tension, as to cause 

 the fracture and consequent destruction of castings before they 

 leave the foundry, sometimes even while in the moulds and at a 

 red heat. 



These reasons, added to the difficulty of cutting and drilling 

 pipes made from it, render iron of this quality unfit for the pipe 

 founder's use. 



In order to obviate the difficulties which attend the manufac- 

 ture of pipes from either very soft or very hard iron, the practice 

 of mixing the two kinds in the furnace is often resorted to, with 

 the intention of producing such as is supposed to possess the 

 proper character. The practice is however objectionable, as the 

 mixture is never chemically perfect. 



It is a fact well known to chemists, scientific iron masters, and 

 steel manufactures, that the chemical union of carbon and iron 

 can only be effected by cementation at a moderate red heat, and 

 that they cease to combine when the temperature of the iron 

 reaches the melting point. This being the case^ if hard iron and 



