CONTRIBUTIONS OF METALLURGY 255 



cast is called, with a cross section about four times that of the 

 cannon itself, and then forge it down with extremely costly 

 presses and with a very great outlay of energy, into its final 

 shape? Why do we not proceed as in making a statue, and 

 cast the molten metal directly into a cannon of the exact size 

 and shape in which it will be used ? In short, why are cannons 

 forgings instead of being simply castings ? " 



Partly from copying blindly the procedure which was neces- 

 sary when cannons were not cast from the molten as a single 

 piece of steel, but were built up from a large number of 

 lumps of wrought iron, which had to undergo a great amount 

 of forging in order to weld them together. Apart from this 

 minor and valid reason is that the kneading under the hy- 

 draulic press closes up any small cavities which form in the 

 solidification of the molten mass. But the chief motive is 

 that this kneading may lessen the extreme heterogeneousness 

 which such a cast mass necessarily has, as I will now show. 



Solidification is an extremely complex process of differentia- 

 tion. This differentiation is familiar, though not by so long 

 a name, to every country bred boy, who knows that if a vessel- 

 ful of cider is frozen half way through, the half which re- 

 mains unfrozen, surrounded by the frozen part as by a jacket 

 of ice, is far more stimulating and joyous than the original 

 fermented juice of Eve's fruit. There is no more alcohol 

 present in the mass taken as a whole than when we started, 

 but that which is present has been concentrated in the un- 

 frozen " mother liquor," because of this differentiation in 

 freezing. The earliest frozen layers in the act of freezing re- 

 ject part of their alcohol content and thus concentrate it in the 

 mother liquor. 



A parallel process occurs in the solidification of a steel 

 ingot. The carbon as well as the harmful impurities, phos- 

 phorus and sulphur, which we have failed to remove com- 

 pletely in the purification of the steel, become concentrated 

 progressively during solidification in the remaining molten 

 metal. Each successive layer of solid steel, deposited from 



