involved in the Construction of Artillery. 241 



also from their form, as yet imperfectly determined, it is scarcely possible to 

 observe visually, any determinate arrangement of the crystalline axes, in good 

 gun-metal, with reference to the contour of the mass ; but some experiments 

 which have been made abroad as to the relative tenacity of bars of gun-metal, cut 

 from the same gun, in two different directions, and broken by transverse strains, 

 appear to indicate distinctly that tlie molecular or crystalline arrangement of 

 gun-metal in cannon develops itself in precisely the same manner as that of 

 cast-irpn in guns, or generally of all crystalline fusible bodies. Thus, when 

 bars of equal section were cut from the gun, in a direction parallel with the axis 

 of the piece, and others in directions radial to the axis, or perpendicular to the 

 former, and both broken ; the tenacity of the latter exceeded that of the former 

 in about the ratio of 30 : 25. If the principal axes of the crystals be in the 

 lines of least pressure, they must be found arranged radially to the axis of the 

 gun ; the maximum cohesive resistance of all metals is in the direction of the 

 principal axes of the crystals (as, for example, in the line of the fibre or acicular 

 crystals of rolled wrought-iron) ; but in this case we find the greater cohesive 

 resistance is in the direction radial to the axis of the gun ; we may, therefore, 

 conclude, as the structure is crystalline, that the principal axes are in the same 

 direction. 



187. Gun-metal, therefore, comes within our general law as to its molecular 

 constitution, but in proportion as the quality of the metal is bad, its substance 

 boursouffle, and filled with microscopic vesicles of gases liberated in casting or 

 cooling, and rendered ununiforin by the segregation of anomalous alloys, &c., 

 in the same proportion will it be difiicult or impossible to observe any normal 

 arrangement whatever of its particles. 



188. When bronze guns are burst in proof or service, or broken by the stroke 

 of shot, a general and often strongly marked tendency to crystalline arrangement, 

 radial to the axis of the piece, maybe observed. We cannot, however, rest any 

 decisive conclusions upon fractures so produced, inasmuch as the crystalline 

 axes are changed, and often abnormally everted, by the action of internal compres- 

 sions and extensions, beyond the elastic limits of the material, producing effects 

 similar to those hereafter treated of as occurring in wrought-iron, at ordinary 

 temperatures, when exposed to blows, or other violent strains or changes of 

 external form, beyond the limits of recovery. 



