involved in the Construction of Artillery. 371 



remaining at temperatures, however elevated, pasty and viscid. It contains less total carbon 

 than the preceding, and is, in fact, an approach to imperfectly developed wrought-iron. 



Fig. 5, a portion of a large pig of No. 1 Scottish cast-iron; soft, very dark gray in 

 colour, very fusible, containing a very large proportion of uncombined carbon in the state 

 of suspended graphite, diffused in scaly or micaceous crystals throughout the mass, upon 

 which it confers its peculiar form of large, pretty uniform, but irregular and ill-developed 

 crystallization, with its dark gray metallic lustre relieved here and there by light reflected 

 from the flat faces of spangle-like crystals, some of which often can be separated, and blown 

 away from the surface. This is the other extreme end of the series of cast-irons ; the most 

 fusible and liquid when melted — the least rigid and tenacious, and the softest when cold. 



Cast-irons, produced directly in the blast furnace, with properties intermediate between 

 Fig. 6 and Fig. 5, and passing by insensible degrees from one to the other, or produced 

 by mixture in fusion of the two, constitute the vast mass of the castings of commerce for 

 all purposes, and the pig-irons known as Nos. 2 and 3 ; and of these mixtures, cannon, &c., 

 are frequently cast. But — 



Fig. 7 represents a portion of the fractured surface of a mass of " finely mottled cast- 

 iron," of the proper texture and quality for casting ordnance, as obtained directly from 

 the smelting or blast furnace, and at once run into guns. If run into pigs, and again 

 melted for guns, it approaches in the process either towards No. 6 or No. 5. 



This mottled iron may be imitated by mixing Nos. 6 and 5, with more or less success, 

 in proportion to the skill and tentative knowledge of the founder, and the qualities of the 

 pig-irons employed ; but i/ie physical properties of the cast-iron so produced are totally diffe- 

 rent from those of mottled iron prepared in the smelting process, by u-hich, alone, fineness of 

 mottle can be insured. The fineness suited to guns is shown in the figure, to natural size. 



Fig. 4 represents the form of development of crystals in octohedrons, frequently found 

 lining the walls of " draws," or other internal cavities in castings of iron. That figured was 

 in fine mottled iron. The subject is referred to in Chaps. 5 and 6 of text. 



Figs. 1 and 2 represent the two normal extremes of molecular structure of wrought- 

 iron of good quality. 



Fig. 2 is a fragment fractured from a large mass of forged or steam-hammered iron : it 

 consists of large crystals ; some, in the specimen drawn, as large in surface as a fourpenny 

 piece, with distinct cleavage in planes, generally perpendicular to the cooling surfaces or 

 contour of the mass, and, therefore, generally parallel to planes of fracture. 



Fig. 1 is a portion of a round bar, of 2 inches diameter, rolled out of iron identically 

 the same in quality with Fig. 2, the bar being " nicked" on one side, and then broken and 

 bent back by blows to the form figured. Its structure consists of perfectly uniform, 

 straight fibre, or crystals, all parallel to the axis of tlie cylindric bar. This is the other 

 normal extreme. 



