than in ordinary granite ware or tinware. Further, it is not attacked by weak acids such as those found in any fruits used as food. Moreover, it is light, does not tarnish readily, and is easily cleaned. Anything alkaline

Fig. 67. A thermit crucible ready for use in mending a broken casting. (By courtesy of the Goldschmidt Thermit Co.)

in character cannot be cooked in aluminum vessels but none of our food is alkaline. In the finely powdered form aluminum is mixed with linseed oil and used as a paint for metallic objects to which it adheres well. Its use in this way is seen in many of the penny-weighing

machines, mail boxes, steam and hot water radiators, and other familiar objects. Very considerable quantities of powdered aluminum are used in a patented article called "thermit." This consists of ferric oxide and

Fig. 68. A thermit crucible in operation, mending a broken casting. (By courtesy of the Goldschmidt Thermit Co.)

aluminum intimately mixed; for use it is put into a crucible of suitable size, conical in shape, as shown in Fig. 66, and a small amount of powdered magnesium placed on top as kindling. When everything is ready the magnesium is lighted. The heat thus obtained is sufficient

to start the chemical action between the aluminum and ferric oxide, Fe,O,. Once begun it is self continuous from the heat liberated. In a short time the aluminum has become aluminum oxide and the iron is free, in a molten condition with a temperature over 1,500°. In fact, it is said that a temperature of 3,000° is often reached in the process. By substituting the oxides of other metals, such as chromium or vanadium or manganese, it is possible to obtain these rare metals in a pure condition. The process is used for welding almost everything of any size, made of iron or steel. Thus are mended broken drive wheels for locomotives, propeller shafts for great engines, rails in street railway systems, and the like. It is frequently used on shipboard for repairs not easily made otherwise. The aluminum oxide obtained as a byproduct, is often used for the manufacture of synthetic rubies and sapphires already mentioned and as an abrasive. Another extensive use for aluminum is in the manufacture of steel. There is a tendency for molten steel, made as much of it is, to retain oxygen or other gases within the mass. This is probably due to the viscosity. When such ingots of steel are made into rails these "air holes" form weak places and probably are occasionally the cause of railway accidents. On account of the tendency of aluminum to combine with oxygen readily at high temperatures, a certain amount of the metal is added to the steel: the aluminum combines with the oxygen, forms a kind of slag and rises to the top.

7. Alloys.-Aluminum forms several alloys of value. Magnalium, containing about 2 per cent of magnesium, has already been mentioned. (See p. 381.) Aluminum bronze is prepared in two varieties, one with a very small percentage of copper, which is even whiter than pure aluminum and resembles silver closely except in

density. It is used extensively in novelty articles and occasionally in such kitchen utensils as teapots. The other variety, with copper as high as 90 per cent, somewhat resembles gold in color and is frequently used in making watch cases as well as a great variety of novelty articles.

8. Alums. An alum is a double sulphate of a univalent. and a trivalent metal. There are many of them, but the most common is potassium aluminum sulphate, K2SO4.Al2(SO4)3.24H2O. Almost as common is ammonium alum in which ammonium has taken the place of the potassium in common alum. Sodium being univalent, enters into the composition of several alums; chromium and trivalent iron, likewise, may take the place of aluminum in common alum. They are all hydrates and contain the same amount of water; they are all also isomorphous, that is, they all crystallize in the same shape. "Burnt" alum is obtained by heating alum to expel the water of combination. It is a mild caustic and is sometimes used medicinally in that way, especially for ulcerations of the mouth.

9. Uses of Alum.-Mention has already been made that alum is used as a coagulant in purifying muddy waters for city supplies. More often, not alum, but aluminum sulphate is used, although it is generally referred to as alum. Its reaction with the milk of lime is

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3Ca(HO)2 + Al(SO1); → Al,(HO) + 3CaSO4. Likewise, alum has been spoken of as an ingredient of baking powders. More often, here also, aluminum sulphate is used instead of real alum. It is supplied the baking powder factories under the trade name of C.T.S. meaning “cream tartar substitute." The chemical action has already been noted on page 329.

10. Aluminum Hydroxide, Al(HO). This is easily

prepared in the laboratory by treating a solution of alum with ammonium or sodium hydroxide, taking care not to use the latter reagent in excess. Sodium carbonate may be substituted for the alkali. It is a white gelatinous precipitate, in many ways a very interesting compound chemically. Like zine hydroxide, it ionizes both as a base and as an acid, thus.

A1(HO), ⇒ Al + (HO), (HO), (HO),

As a result aluminum hydroxide is very soluble not only in acids, with which it forms aluminum salts with aluminum as the positive ion, but is also soluble in bases, with which it forms aluminates, in which the negative ion is -AlO3. To illustrate,

Al(HO) 3+3HCI → AICI, + 3H2O,

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H1AlO + 3NaHO → NaAlO3 + 3H2O.

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Its position in the table, near the acid forming elements, would lead us to expect such behavior.

On account of its gelatinous character not only has it the power of clarifying muddy waters, but also of removing colors from solutions. Thus, if an alum solution, deeply colored with some dye, such as carmine, has a little alkali added to precipitate the aluminum as hydroxide, in settling, the aluminum hydroxide will carry with it practically all the coloring matter. Such precipitates dried and ground in oil are sold as tube paints for artists under the name of lakes, of crimson and other brilliant colors. It is the same principle that makes it a good mordant. Precipitated within the fibers of the cloth it holds or fixes the color. Canvas for tents and other fabrics are sometimes waterproofed by this compound. Treated first with aluminum chloride or some similar compound