involved in the Construction of Artillery. 359 



verted more or less perfectly into the former, by repeated fusion in direct contact with fuel 

 and blast; and it has been equally well known that either white or gray iron may bo 

 obtained at will, and by a first or single fusion, from the smelting furnace. 



For nearly thirty-five years, the nature of these changes has been fully understood, 

 through the researches of Karsten, by whom it was proved that gray cast-iron (Nos. 1, 2, 3) 

 contained carbon in two states, chemically combined and mechanically diffused, the latter 

 as crystals or scales of graphite; and that white cast-iron (No. 4) contained carbon in but 

 one, viz., wholly in combination with the iron, — the extreme case being that of the Spie- 

 geleisen (Fig. 3, Plate v.), in immense, well-defined crystals, which contain above 5 per 

 cent, of combined carbon, — somewhat less being found in Fig. 6, the more usual fracture 

 of lamellar (No. 4, white pig-iron); becoming mixed with uncombincd carbon, as graphite, 

 in the mottled iron (Fig. 7) ; and having its largest proportion of diffused graphite in very 

 dark gray (No. 1, pig), possessing fractures more or less resembling Fig. 5. 



Karsten proved, that any one of these varieties of cast-iron could be converted by suit- 

 able metallurgic treatment into any other, and that, as respects the conversion of gray cast- 

 iron into white, the process was, to a greater or less extent, the inevitable result of every time 

 the gray metal was melted and cooled, — that it was dependent simply on two conditions : — 

 1°. The deprivation of graphite in the furnace, due to the proportion that should be 



given to air-blast and fuel. 

 2°. To the fact that in the act of consolidation a certain proportion of the whole 

 of the suspended graphite was exuded, i. e. forced out to the surface of the 

 cooling mass, by the crystallization of the whiter portions, whose carbon is 

 combined. 



Now, it follows as matter of course from these well-known facts, that, as perfectly white 

 cast-iron has at once the highest cohesion and the greatest brittleness, while properties the 

 reverse belong to the darkest gray graphitic cast-iron, — some mixture of the two qualities 

 {not of the tioo irons) must give the best material for gun-founding, or for any other mecha- 

 nical purpose, in which the highest product of tenacity and toughness is demanded. And 

 in this consists the value of " mottled iron" (Fig. 7, Plate v.) for cannon. 



It also is obvious, that a more or less perfect approach to such a mixture may be made 

 by repeated melting and cooling, up to a certain point, of any gray iron ; but the number of 

 meltings and coolings necessary to effect this will differ, not only with the original gray 

 iron tried, but with the conditions of the cupola furnace in every consecutive melting, 

 and with the conditions of cooling at every casting, so that probably no two series of expe- 

 riments could be possibly made, that should give co-ordinate results, or that would be apj^li- 

 cable to any other make of iron, or to any other cupola, fuel, and blast. Moreover, the quan- 

 tity of graphite eliminated at each cooling is greater, in some proportion, as the cooling is 

 more rapid. The trial, therefore, that shall give the number of meltings producing the 



3a2 



