234 



Mr. Mallet on the Physical Conditions 



cast; and this must be true, however well or ill founded M. Briche's opinion 

 may be, of the advantage of a previous high temperature in the melting furnace 

 to induce perfect combination of the metals. 



177. The more rapidly, then, the mass can be cooled, the better. In the case 

 of cast iron guns we found this to be so likewise, but from the properties of that 

 metal we cannot push it far ; the case, however, is very different with gun-metal, 

 where sudden cooling, as sudden as when the red hot mass is quenched in cold 

 water, is attended with this singular and opposite result, that the metal is thereby 

 rendered softer, tougher, and more malleable, — a discovery due to M. Darcet, 

 and subsequently pursued and experimented on by Dussausoy, the results of 

 some of whose researches are given in the following Table, showing the effects 

 of this tempering (trem.pe) or sudden cooling, upon five principal alloys of copper 

 and tin, embracing a large range. 



These results seem to indicate, that within the limits of the gun-metal pro- 

 portions, sudden cooling does not injure, and may even increase the tenacity 

 (absolute cohesion), and that probably it diminishes the density and the hardness 

 of the alloy. Dussausoy indeed ascertained that the remarkable alloy (Cug + Sn), 

 which is exuded by gun-metals in consolidating, is that which is most im- 

 proved by sudden cooling (trempe) ; that its tenacity is thus invariably increased, 

 no matter what may be the mass or thickness of the casting. There are 

 sufficient grounds for believing that where all the requisite conditions are 

 attended to, increased density and tenacity, with homogeneity of composition, 

 will result from the rapid cooling of gun-metal, whatever may be the mass of 

 the castings. 



178. As, therefore, in the casting of guns we may neutralize a diminution of 

 density by increasing the statical pressure of the " head of metal," and can in- 

 crease the hardness, by the addition of more tin, it seems well worthy of careful 

 trial whether a better and much more homogeneous result might not be obtained 



