the cloth is heated by steam which hydrolyzes the aluminum compound, forming aluminum hydroxide within fibers of the cloth. Upon drying it becomes waterproof. Some papers are sized by aluminum hydroxide. To the pulp aluminum sulphate and rosin soap are added; through hydrolysis aluminum hydroxide is precipitated in the pulp. When dry it is run between hot rollers. This melts the rosin and gives a surface to the paper while the aluminum hydroxide fills the pores, so that ink is not taken up readily.

Exercises for Review

1. Name the members of the aluminum family and give location in the table.

2. State what is true of the abundance of aluminum. Name some familiar natural compounds.

3. What is emery? The ruby? Kaolin? A synthetic stone?

4. Describe the preparation of aluminum.

5. Give the characteristics of aluminum.

What is a flux?

6. Why is aluminum suited for cooking vessels? What advantage has it over copper?

7. Describe the use of thermit.

8. Name some alloys of aluminum and give uses.

9. What is an alum? Name two. What is burnt alum?

10. What is the meaning of the term isomorphous ?

11. Why is aluminum hydroxide soluble in both acids and bases? What other hydroxide has been seen to have the same property? 12. How are painters' lakes made?

13. What is meant by sizing paper?

14. Complete the following equations,

Al,(SO,), + KHO →,

3

Al(HO), + H,SO,

Al, (HO) (heated) →

Al,O, (electrolyzed)

Fe,0, + Al →,

Al + 02 →.

Note. The student will use the amounts as needed in the above

equations.

1. Metals of the Group.-The only common metals belonging to this group are tin and lead, the former with an atomic weight of 119 and the latter, 207.1. Their position in the table should show a valence of four and their higher oxides indicate this.

2. Occurrence of Tin.-The oldest tin mines of the world are those of Cornwall, England. It is said that the ancient Phoenicians obtained their supplies from this source. These mines now at great depth and extending out under the ocean are still in operation but produce scarcely 10 per cent of the world's output of the present time. Most of that used comes from the

East Indies where it is found as tin oxide, SnO2, called cassiterite. It is the same ore as that from the English mines.

3. Characteristics.-Tin is a white metal with a melting point of 232° C. It is soft, very malleable and has a specific gravity of 7.3. It is crystalline in structure, but much less so than antimony or bismuth. Like phosphorus, sulphur and some other elements already studied, tin also occurs in an allotropic form not often seen. Kept continuously below 20° C. it sometimes changes to a gray powder, expanding to such an extent that its specific gravity is only 5.8. Tin is not tarnished in the air or attacked by any of the organic acids. It decomposes hydrochloric acid with the evolution of hydrogen, also concentrated nitric, with the formation of nitrogen. peroxide. Hot sulphuric acid gives off with tin sulphur dioxide, as is usual in such cases.

4. Uses of Tin.-Because of its permanence in the air tin is used extensively in protecting iron in what is called "tin plate." It is made in a manner similar to that used for galvanizing iron. Clean, heated sheets of steel are dipped into molten tin and upon removal a coating adheres. However, as iron is more electropositive than tin, if the coating is scratched so as to expose the iron the corrosion is then rapid. The reverse is true with galvanized iron. Of tin plate are made the so-called "tin cans" used so extensively in preserving various food products. During the year 1919 more than six billion tin cans were used in the United States in the various canned-food industries. In the infancy of food preservation in this manner, each canner made in his own establishment by hand labor the cans needed. A good workman could not produce over 150 per day; now by machine they are turned out at the rate of about

one per second. Heavy tin plate is used for roofing and gutters. Some years ago cooking vessels of tin plate were common, but they have been largely replaced by granite ware, which is more serviceable, and more recently by aluminum. Copper cooking vessels used in large eating houses, such as the Harvey system, are tinned on the inside to prevent attack by organic acids. This has to be done somewhat frequently, but the process is simple. The vessel is thoroughly cleaned as if for soldering, is heated to the melting point of tin and then the powdered metal rubbed on the surface to which it adheres. The flux used in cleaning the copper, usually ammonium chloride, prevents oxidation when the vessel is heated. Common brass pins usually have a thin coating of tin to prevent their tarnishing in the air.

5. Alloys. Many valuable alloys of tin are used in the arts. Bronze is composed of copper and tin, sometimes with zine added; brittania and pewter have already been mentioned, as have certain fusible alloys. (See p. 289.)

Soft solder contains tin and lead in equal proportions. Tin is also an important component of the most common dental amalgams.

6. Compounds. Tin forms both stannous and stannic salts, represented by the chlorides, SnCl, and SnCl. The stannous chloride is an unsaturated compound and therefore a reducing agent. Added to a solution of mercuric chloride, first a white precipitate of calomel is obtained; upon warming or adding more stannous chloride a gray or black precipitate is formed consisting of finely divided mercury or a mixture of it with mercurous chloride. These two equations represent the reactions occurring,

The experiment serves as a test for mercuric compounds, or reversed, for a stannous salt. On account of this reducing power of stannous chloride it is used with gold chloride in toning printing out papers as already described.

There are two oxides of tin, stannous, SnO, and stannic, SnO. The latter is the compound found native. It may be obtained by treating tin with concentrated nitric acid and heating the white powder obtained. is white when cold, but distinctly yellow when hot. Stannic acid, which is a white gelatinous compound, is used extensively in weighting silk goods. The chlorides are used somewhat as mordants.

7. Occurrence of Lead.-Galena, PbS, is the only important ore of lead. It is a lustrous, dark gray mineral crystallizing in cubes, with cubical lines of cleavage. It is associated with many of the zine deposits in southwest Missouri and with silver and other metals in many of the Western States.

8. Characteristics of Lead.-Lead is a dark gray metal with a specific gravity of 11.38; it is soft and malleable but of little tenacity. It tarnishes somewhat readily in the air, but the coating adheres firmly and serves to protect the metal from further oxidation. The melting point is 325° C. Being just above hydrogen in the electromotive series it has little power of replacing that element in acids. With nitric acid it reacts as do most of the metals to form nitric oxide, thus,

3Pb+ 8HNO→3Pb(NO), + 4H0 + 2NO.

Sulphuric acid, boiling hot and much concentrated produces sulphur dioxide, thus,