involved in the Construction of Artillery. 171 



better than what is attainable by the admixture of a proper selection of different 

 cast-irons alone, — though well deserving a much more careful course of experi- 

 ment on the part of the gun-founder, than has yet been devoted to it. Much stress 

 has been laid by some, too, upon the melting being performed in air furnaces in 

 preference to cupolas urged by blast. The only real difference as respects good- 

 ness of result, however, seems to depend upon temperature; if this be the same, 

 the results are the same with either form of melting. 



69. Usually, however, the temperature of the cupola is vastly higher than that 

 of the air furnace, and its consequence is the formation of an alloy of the bases 

 of the alkaline and earthy metals with the cast-iron, by which its tenacity is 

 always seriously reduced. These alloys, when present largely in pig-iron, are 

 always indicated by a peculiar whitish pallor of the fresh fracture, and are only 



formed at intense heats, at which also the micaceous plates of uncombined car- 

 bon become developed as graphite, upon the largest scale, a fact first pointed out 

 some years since by Schafhaeutl. Unnecessary heat effusion then injures the 

 quality of the metal, as unnecessary heat of " pouring" injures the quality of the 

 casting. It does this in two ways, by the introduction of foreign earthy and 

 all^aline bases, which greatly reduce the cohesion, and far more by the great in- 

 crease of surface produced by extreme elevation of temperature, in the disse- 

 minated plates of graphite. These, scattered through the mass like mica or 

 hornblende in granite, present at their innumerable planes of cleavage almost 

 no cohesion ; but these planes are, in accordance with the general law of ar- 

 rangement in the " planes of least pressure," found mainly to coincide in paral- 

 lelism with those of the crystals of the iron itself (i. e. the carburet of iron which 

 constitutes the metal of cast-iron chiefly), so that the total deterioration of strength 

 due to smelting at an extremely high temperature is very great, and this is in fact 

 the secret of the much discussed and unquestionable inferiority of hot-blast iron 

 over cold ; nothing more than the elevated temperature induced in the bias fur- 

 nace. All cast-iron, in its progress towards wrought-iron in the " puddling" pro- 

 cess, passes through an intermediate stage, in which it is more or less perfect 

 cast-steel ; and the Styrian steel (Stahleisen) is produced direct from the " finery 

 pig" merely by an adroitly managed puddling, stopped at the proper moment. 



70. I shall conclude this portion of the subject by the following Table, princi- 

 pally derived from my Report (Trans. Brit. Ass., 1840), in which the general 

 characteristics and working qualities of the more important "makes" of Bri- 

 tish cast-iron are combined and systematized : — 



