240 Mr. Mallet on the Physical Conditions 



cylinders had been screwed, through which the vents were bored, as compared 

 with guns in which the vents were bored dii-ectly out of the solid cast-iron, and 

 the evil is attributed to the metal of the gun removed, in addition to that of the 

 ordinary vent, to make way for the copper, and to the excess of the expansion of 

 the copper above that of the cast-iron, which grips it all round, when both are 

 heated by firing, and thus produces a strain upon the gun. It will, how- 

 ever, depend upon circumstances whether the gun shall yield to the copper, or 

 the latter to the gun, to such an extent, as to make the tension of the gun in- 

 appreciable. 



20. — Molecular Constitution of Bronze, or Gun-Metal, in Cannon. 



185. Guu-metal of the finest quality, when freshly broken, presents a beau- 

 tifully fine matted fracture, nearly uniform, and of an even gold colour, with a 

 few fine specks of brilliant light, uniformly disseminated. These are the facets 

 of larger crystals ; examined with the microscope, the whole mass is found to 

 consist of extremely minute crystals. 



Though the mass is crystalline, however, it is highly ductile, and unless the 

 fracture be produced by a direct tensile strain, applied suddenly as an impact, 

 the forms of the crystals are distorted and bent in its production, and the cha- 

 racter of the fracture becomes changed and deceptive. 



The size of the crystals is always very small, and their form unpronounced 

 when the metal is good ; but very minute changes in the proportions of copper 

 and tin, combined probably with some conditions as to fusion, temperature, etc., 

 as yet unascertained, occasionally give rise to a large development and singular 

 regularity of crystalline structure. 



This, however, is never developed to the extent frequently observed in the 

 alloys of copper and zinc, some of which, chiefly those between (2 Cu -f Zn) 

 and (Cu + 2 Zn), Table x., may, by peculiar treatment, be obtained in crystals, 

 as large or larger than those, found even in the hardest crystalline cast-iron 

 (Speigeleisen). In such cases the principal axes of the crystals are found 

 arranged according to the general law, in the lines of least pressure within the 

 mass, on its consolidation. 



18G. From the minute size of the crystals of gun-metal, and possibly 



