IRON 



979 



iron varies from tin white to greyish white; it is very brittle, cracking easily, even 

 by change of temperature; it is extremely hard, sometimes even more so than 

 hardened steel, so that it will resist the strongest file, and scratches glass easily. 

 Fracture sometimes laminar, sometimes lamino-radiating, sometimes finely splintered, 

 sometimes dense and conchoidal. As the fracture changes from laminal to conchoidal, 

 the colour likewise varies from white to greyish. Mean specific gravity, 7'5. Ex- 

 pands less than grey cast iron when heated, cannot be welded, because it becomes 

 pasty at the very lowest welding heat. When heated to the melting point it does 

 not suddenly pass into the fused state like, grey pig-iron, but is converted before 

 fusing into a soft pasty mass. In this variety of pig-iron the whole of the carbon is 

 united to the iron ; it is never used for casting, but always for conversion into 

 malleable iron. The bright iron obtained from spathic iron-ore contains the largest 

 proportion of carbon (5'3 per cent, according to Karsten). The white iron resulting 

 Irom derangement flows imperfectly, and darts out in casting abundance of white 

 scintillations ; it fixes very quickly, and on cooling exhibits on its surface irregular 

 asperities, which make it extremely rough ; it is exceedingly hard, though it is easily 

 broken, the fracture being radiated and lamellar ; the bar-iron it affords is of inferior 

 description. This kind of iron is always produced when the furnace is carrying a 

 heavy burden of forge cinders containing sulphur and phosphorus. 



Thus there are two distinct kinds of white cast iron : 1st. That obtained from ores 

 containing a large proportion of manganese crystallising in iron plates ; this variety 

 is highly prized for making steel. 2nd. That resulting from a heavy mineral burden, 

 or from a general derangement of the furnace, or from the rapid chilling of fused grey 

 iron crystallising in small plates ; both are hard and brittle, the first more so than the 

 last. Cast iron, which by slow cooling is grey, becomes white when it is cooled 

 rapidly ; on the other hand, when white iron is melted and allowed to cool very 

 gradually, a portion of the carbon crystallises out as graphite, and grey cast iron is 

 produced. 



In some iron works six varieties of pig-iron are recognised, which may be classified 

 thus : 1. First foundry iron, large crystals; 2. Second foundry iron, large and small 

 crystals mixed ; 3. Dark grey, all small crystals ; 4. Bright grey ; 5. Mottled ; 

 6. White, verging on mottled. 



The subjoined table exhibits the composition of some different varieties of Conti- 

 nental, English, and American crude irons. The methods of determining the various 

 elements which nearly always accompany cast iron, are given at the end of this article. 



Besides the substances enumerated in the above table, other metals, such as copper, 

 arsenic, chromium, titanium, cobalt, zinc, tin, aluminium, and the metals of the alkalis 



3R2 



