radium came about in the following way:- Becquerel found, in 1896, that when uranium compounds (e. g., pitchblende, a uranium ore) are placed near photographic plates which are protected from light, they affect the plates like light itself. He ascribed the effect to active rays ("Becquerel rays ") given off by the uranium. The simplest way to test for the presence of these rays, and for their amount, is to hold the material to be examined near a charged, insulated electroscope. If the material is "radio-active," the air becomes a conductor, and the electroscope is discharged. The time required measures the radioactivity" of the substance. Professor and Mme. Curie, of Paris, removed the uranium compound from pitchblende, and found that the residue was still very active. By separating it carefully into its parts they obtained a small amount of barium chloride, and by analysis of this they secured a minute quantity of a new salt, radium chloride. This was more than 1,000,000 times as active as pitchblende. The properties of radium compounds place the element with the alkaline-earth metals, which are bivalent. The equivalent weight of the element was found, by analysis of the chloride, to be 113.2; hence the atomic weight is 226.4. In the periodic table it fills the space below barium. Radium compounds have the formulas RaCl2, Ra(OH)2, etc. All give off heat constantly, are self-luminous, and are strongly radio-active. In 1910 radium carbonate, RaCO3, was treated with hydrazoic acid (HN3), and converted into the hydrazoate, Ra(N3)2. When this was heated in a vacuum, it broke up into radium and nitrogen. The radium had a metallic luster, and, like its salts, was radio-active.

The phenomena of radio-activity are explained by the assumption that the rays consist of particles, called electrons, or corpuscles, which are given off by the radio-active substance. Radium compounds also give off a gaseous material called radium emanation, or niton. This is condensed by liquid air. It belongs to the argon family, has the atomic weight 222.4,

and is in the same periodic series with radium, etc. When niton is examined with the spectroscope, it shows the spectrum of helium. Thus it appears that radium is constantly being formed by the "decay" of uranium, that niton is formed by the decay of radium, and that niton breaks down into helium, the element of lowest atomic weight in the argon family. The discovery that niton gives helium belongs to Sir William Ramsay, of London. He found, further, that when cupric nitrate is enclosed with niton, some lithium nitrate is formed. The copper is said to be degraded" to lithium, the element of lowest atomic weight in the alkali family. Ramsay has also found that the elements of the carbon family, i. e., thorium, zinc, titanium, and silicon, are degraded" by the emanation into carbon. We must conclude, therefore, that some of the elements decompose, giving others of lower atomic weight. This does not do away with the conception of the atom, but, on the contrary, enlarges it.

444. Exercises.

1. How would you prove that some calcium nitride is formed when calcium burns in the air?

2. Magnesium liberates the hydrogen of ammonia; is ammonia, therefore, an acid?

3. Would you expect barium sulphate to be soluble in concentrated sulphuric acid (cf. § 267)? Name the product, and write the equation.

Does carbonic acid react with barium carbonate? Write the equation.

4. Write the equation for the preparation of the oxide of each of the alkaline-earth metals from its nitrate. Name these oxides in the order of their reactivity with water, beginning with the most active.

5. From the relative solubilities of barium carbonate and sulphate (cf. Appendix viii) would you expect to change the

latter into the former by boiling it with sodium carbonate solution? How could you separate a mixture of strontium and barium sulphates?

6. How could you separate the metal ions in a mixture of strontium chloride and magnesium chloride? Barium nitrate and potassium nitrate? Write the equations.

7. Write the partial and complete equations for the formation of magnesia alba from magnesium chloride.

CHAPTER XXXII.

ZINC, CADMIUM, AND MERCURY.

445. The Zinc Group. The elements zinc, cadmium, and mercury are members of the second group of the periodic system. They are, however, much more closely related to magnesium than to the calcium group proper. Zinc and cadmium are very much alike, and are usually found together in nature; but mercury differs from them in some important respects. In the vapor state the molecules of all three contain one atom each (cf. § 142).

446. Zinc.-Zinc generally occurs in combination as the sulphide, ZnS, the carbonate, ZnCO3, and the silicate, Zn2SiO4. The metallurgy of zinc is simple. Its ores are generally roasted to convert them into oxide, ZnO, and the oxide is reduced by charcoal.

The reduction of zinc oxide takes place in retorts or furnaces; from these the metal distills over into condensers. At first the vapors condense as a powder (zinc dust); afterwards the metal condenses as a liquid, and is cast into plates and bars. The zinc thus obtained (called spelter) is not pure. Pure zinc is obtained by the electrolysis of zinc chloride.

447. Properties. -Zinc is a white, hard, and lustrous metal. In dry air it does not change; but

ordinarily its luster is soon dulled by a covering of basic carbonate. At the ordinary temperature, zinc is brittle; but between 100° C. and 150° C. it can be rolled into sheets and drawn into wire. At higher temperatures it is so brittle that it can be powdered.

Zinc melts at 420° C., and boils at 920° C. When heated much above the melting-point, it burns to zinc oxide. Water does not affect the metal, even at 100° C. Hot solutions of alkali hydroxides react with it, forming zincates and hydrogen (cf. §§ 57, 388, and 474).

Zn+2 KOH → K2ZnO2+H2.

Commercial zinc reacts readily with all the ordinary acids; but the pure metal requires a catalyzer, such as copper or platinum (cf. § 47.)

448. Uses. Zinc is used for the positive plates of electric batteries and in alloys, e. g., brass (cf. § 458), German silver, etc.

Galvanized iron is iron coated with zinc by dipping it into a bath of molten zinc. Galvanized iron resists the action of air and moisture better than tinned iron. It is used for wire netting, corrugated roofing, gutters, water-tanks, etc.

449. Zinc Compounds. Among the important zinc compounds are the oxide, hydroxide, chloride, sulphate, and sulphide. Soluble zinc salts are poisonous.