228 Mr. Mallet on the Physical Conditions 



of M. Briclie, of the Gun-foundry at Strasbourg, is founded (Journal des Mines, 

 torn, vi., an. v. de la Eepublique) — namely, that the higher the temperature to 

 which the melted mass is exposed, the more uniform is the metal when cast. 

 Experiments made in the United States in 1850 render it extremely probable 

 that the evolution of gaseous matter, referred to (see 164, 165), occurs chiefly 

 within the limits of a very high range of temperature, which corroborates the 

 explanation here offered of the phenomena. These experiments also showed that 

 the best results were obtained from castings " poured" at a low temperature, 

 though still a determinate one, beneath which they again deteriorated. This, 

 however, does not conflict with the view that a very high temperature effusion 

 may be advantageous, although analogy renders it improbable. At the best 

 temperature in the American experiments there was no fungoid excrescence 

 driven up over the " rising head," and the large and uniform contraction of the 

 latter in cooling, indicated the great density which the gun-raetal was subse- 

 quently found to possess. 



167. Whatever be the immediate causes determining the segregation of the 

 normal alloy, the separation in any gun-metal having the general formula 

 (Cu;t + Sn,) may take any one of three forms, namely — 



(Cu^, -t- Suj,,) -f Cu,; 

 (Cu^, -f Suj,,) -1- Sn,; 

 (Cu,, -fSn) -j-(Cu + Snj„); 



or more than one of these together. 



168. Alloys, of precisely definite atomic proportions, generally present the 

 best pronounced and most constant physical properties. It has been doubted, 

 however, whether this be so in the case of the gun-metals, the very best of 

 which are found within the limits (given in the preceding Table vn.), viz., 

 between 9 and 12 by weight of tin to 100 of copper. 



On examining more closely the atomic constitution of these four alloys, 

 however, we shall find reason to adhere to the conclusion, that the most defi- 

 nite alloy makes the best gun-metal ; and to remark that tlie curious segrega- 

 tion into two or more different, but still definite alloys, is probably intimately 

 concerned with its toughness and elasticity ; that, in fact, the gun, with the 

 central portions so rich in tin bored out, consists of a comparatively soft and 



