involved in the Construction of Artillery. 



223 



158. In gun-metal, as in every other material for cannon, while sufficient 

 hardness must be secured to resist longest, the abrasion of shot, and the deflagra- 

 tion of the powder, along with the greatest ultimate tenacity, there must be 

 such a balance, of rigidity and ductility with ultimate cohesion, as shall give 

 the maximum value to the coefficients T^ and TV- 



The hardness and rigidity increase with the proportion of tin ; the ductility 

 and tenacity with that of the copper, but not in any direct ratio in either case. 



159. The specific gravity increases with the copper generally, although M. 

 Briche's statements (Jour, des Min. t. v.) indicate the contrary: — 



On this Dumas has remai-ked, that the true density is masked by the effects 

 of various forms of aggregation in the different alloys. The increase of density, 

 however, is certainly towards the copper, though in no ascertained ratio to the 

 decrease of tin. The fusibility is always greater than that of copper, less than 

 that of tin. The ultimate cohesion is always less than that of the best refined 

 tough copper, but greater than that of tin. The ductility less than that of copper, 

 greater than that of tin. The hardness always greater than that of either. 



160. Few examples are met with, of guns formed of metal in strictly 

 atomic proportion, but alloys are therein found, presenting every formula, from 

 (7 Cu + Sn) up to (83 Cu + 4 Sn). The proportions most approved of in the 

 arsenals of Europe appear to vibrate between 100, by weight, of copper to 9 of 

 tin, up to 100 of copper to 12 of tin. In France, 100 copper + 11 tin, by 

 weight, is the proportion fixed by law, invariably aimed at, and, we shall see 

 reason to conclude, judiciously so. In the United States 100 copper + 12-5 tin 

 is adopted for certain species of guns. 



161 . In common with the great majority of metallic alloys, gun-metal is held 



2 g2 



