Iron in its pure state is malleable, and it is a combination of carbon with iron which produces cast-iron. In addition to carbon the cast-iron in this country con tains silica, lime, magnesia, alumina, oc casionally some of the phosphates and other admixtures ; but iron made from magnetic ores is much purer. The strength of cast-iron depends upon its freedom from impurities, and upon the proportion of carbon it contains. The strongest cast-iron contains about three per cent. of carbon, or, according to Mr. Charles May, when the carbon is in the smallest proportion that produces fluidi ty ; a larger proportion tends to make the iron soft and weak, and a smaller hard and brittle. Mr. Glynn, in his evidence before the Strength of Iron Committee, in London, states, that the strongest iron generally shows a clear gray, or slightly mottled fracture, and he considers that the color indicates the combination of carbon with iron which produces the greatest strength. Mr. Stirling states, that while color is admissible as a test of strength, it is not so of chemical consti tution, Mr though dark colored iron is usually brittle, yet black iron when chil led becomes white, although it must be supposed to contain the same quantity of carbon ; hence, as a general rule, he con cludes that color indicates the treatment to which iron has been subjected, and in some cases only the quantity of carbon.
Mr. May coincides in considering the question of strength to be very much re ducible to the quantity of carbon con tained in the iron, as some of the tender est iron skilfully treated will produce some of the strongest castings. Messrs. Stephenson and Stirling mention that the fluidity of Berlin iron is due to the pre sence of arsenic, and the latter has ob served that manganese mixed artificially with cast-iron, closes the grain, and is an improvement both to cast-iron and steel. On wrought iron the effect of manganese is stated to be to give it the hot-short property, while cold-short is produced by the presence of a small quantity of phos phorus ; and the admixture of arsenic renders wrought iron hard and brittle. IRON Gum. Pig-iron of gray color should be melted in an air-furnace with an intense and rapid fire, for iron guns ; but an alloy of copper and tin is used for brass guns. The first are used on ship board and the latter for field artillery, with a bush of copper, as less fusible by firing, for the touch-hole.
The solid casting is then bored by the revolution of the gun, with an apparatus and steam power. A 24-pounder of iron is 10 ft. long, and weighs 52 cwt. with a bore of 5.824 inches, a ball of 5.547 inches and 8 lbs. of powder. A 24-brass pounder weighs 50 cwt. An Iron 6-poun der weighs 24 cwt. and is 9 ft. long.