No. I has a dark-gray fracture, and is somewhat ductile. No. II is white, brittle, and has a granular fracture. No. III is crystalline, with an iron-gray texture, and may be pulverised under the hammer. No. IV is light-gray. The grain, which is dry and without lustre, is filed with great difficulty. It is very brittle and hard. It has a fine lustre when polished, and is grayishwhite in colour. An alloy of iron and tin is said to be preferable to pure tin for tinning copper vessels. § 187. Iron and Zinc.-These metals unite together to form a series of alloys which are hard, more or less crystalline, and brittle. The difficulty of forming alloys by direct fusion is owing to the high melting point of iron, and the volatile character of zinc. Greenwood states that 7 per cent is the maximum amount of iron that zinc will take up, but Guettier states that he obtained alloys containing 50 per cent of zinc and upwards by pouring zinc down an iron tube into a bath of molten cast-iron. The alloys became whiter and slightly more malleable as the proportion of zinc was increased. § 188. Iron and Aluminium.-These metals unite in various proportions, forming hard, white, and brittle alloys when the aluminium is present in notable quantity. Alloys, known as "ferro-aluminium," have been somewhat largely made of late years, and used for introducing small quantities of aluminium into steel and cast-iron instead of adding the pure metal. When the aluminium in the ferro-alloys exceeds a certain amount (about 17 per cent), the alloy becomes non-magnetic, and like ferro-manganese, is not attracted by a magnet. Aluminium added to molten iron and steel lowers their melting points, increases the fluidity of the metal, and causes it to run more easily into moulds and set solid, without the formation of blowholes. Mr. Nordenfelt adds a small quantity of aluminium to wrought-iron, which lowers the melting point sufficiently to enable it to be poured like ordinary cast-iron. The Cowles Electric Smelting Company are now manufacturing large quantities of ferroaluminium, and they claim that this alloy is perfectly homogeneous, and that when a ladleful of molten steel is poured on a portion of the metal, the aluminium is equally distributed throughout the whole mass. The effect of a small portion of aluminium on steel is to raise its elastic limit and ultimate strength, as well as to impart the properties mentioned above. The effect of aluminium on cast-iron has been studied by Mr. Keep of Michigan,1 who states that the appearance of the fracture is a strong indication of the character of the metal, and that a bar inch square, more than any other size, shows by a change of the grain the effect of varying composition. A smaller bar chills so quickly that it does not give an element sufficient time to exert its influence, while a larger bar holds its heat so long that any tendency towards a white grain is more or less overcome. Mr. Keep commenced his tests with a pig-iron containing 1.27 graphite, 1.71 combined carbon, and 08 silicon, which gave castings full of blowholes. To a portion of this pig-iron ferroaluminium was added, so as to produce a metal containing 25 aluminium, 1.37 graphite, and 2 silicon per cent. The result was a more even grain, with absolutely no blowholes. The silicon had increased, and added to the hardness of the casting. With the addition of silicon alone to the above pig-iron, 1 See Proceedings of American Assoc. for Advancement of Sc. August 1888, and Proceedings of American Institute of Mining Engineers, 1889. a casting was obtained nearly as sound, but not so strong, especially in its resistance to shock. The effect of aluminium on white or mottled pig-iron is to convert combined carbon into graphite, as silicon does, but without weakening effects. When the pig-iron contains its carbon entirely in the form of graphite, there is no advantage in adding aluminium. Mr. Keep states that aluminium of itself does not add to the strength of cast-iron, but, through its influence on the carbon, does increase the strength of the iron indirectly. Now as aluminium causes combined carbon to pass into graphite when the metal is remelted, some of the separated carbon rises to the surface, and is removed entirely from the cast-iron, and the strength of the casting is thus increased. Irons made gray by aluminium are valuable for the making of thin castings, where strength as well as softness is desired. The introduction of aluminium into molten cast-iron is attended with serious difficulty. If ferro-aluminium, broken into small pieces, is introduced into the ladle before the metal is "caught," it will be melted as the first iron strikes it; but if plunged into the iron after the iron is caught, it is likely to chill a coating of iron around it. In this condition the metal swims. Pure aluminium thrown on the molten iron melts at once, and sinks into the metal, but the whole does not seem to remain in the iron, as it does when successfully introduced in the form of ferro-aluminium. Aluminium introduced into the foundry ladle in the smallest quantities causes the iron to boil rapidly, probably because of the particles of graphite which it liberates. This ebullition is so rapid that slag which has risen to the surface is often carried under again by the force of the current. CHAPTER XIII MISCELLANEOUS ALLOYS § 189. Alloys for Calico-printing Rollers and Scrapers. -For this purpose a metal is required that is sufficiently soft to be worked by tools, and hard enough to resist the wear to which it is subjected in practice. Another important desideratum is that the metals should be capable of resisting the corrosive action of the liquids with which they are in contact. Hauvel considers a bronze having the following composition the best material for the rollers: copper 84, tin 14, zinc 2. Another alloy which is used consists of zinc 785, tin 15.8, copper 5.6. The following are analyses by Dépierre and Spiral of the scrapers employed to remove the surplus colour from the rollers : The Société Industrielle de Mulhouse many years ago offered a premium for the best alloy for the above purpose. It must possess at the same time elasticity and toughness, hardness and flexibility, without being sensibly attacked by the chemicals used in printing. Messrs. Dépierre and Spiral classify the alloys for scrapers into three groups: (1) copper, with 95 to 100 per cent of copper; (2) brass, with about 60 per cent copper and 40 per cent zinc; (3) brass, containing |