~Radium and the atomic theory.~ If these views in regard to radium should prove to be well founded, it will be necessary to modify in some respects the conception of the atom as developed in a former chapter. The atom would have to be regarded as a compound unit made up of several parts. In a few cases, as in radium and uranium, it would appear that this unit is unstable and undergoes transformation into more stable combinations. This modification would not, in any essential way, be at variance with the atomic theory as propounded by Dalton. EXERCISES 1. What properties have the alkaline-earth metals in common with the alkali metals? In what respects do they differ? 2. Write the equation for the reaction between calcium carbide and water. 3. For what is calcium chlorate used? 4. Could limestone be completely decomposed if heated in a closed vessel? 5. Caves often occur in limestone. Account for their formation. 6. What is the significance of the term fluorspar? (Consult dictionary.) 7. Could calcium chloride be used in place of barium chloride in testing for sulphates? 8. What weight of water is necessary to slake the lime obtained from 1 ton of pure calcium carbonate? 9. What weight of gypsum is necessary in the preparation of 1 ton of plaster of Paris? 10. Write equations to represent the reactions involved in the preparation of strontium hydroxide and strontium nitrate from strontianite. 11. Write equations to represent the reactions involved in the preparation of barium chloride from heavy spar. 12. Could barium hydroxide be used in place of calcium hydroxide in testing for carbon dioxide? CHAPTER XXV THE MAGNESIUM FAMILY =========================================================================== |SYMBOL |ATOMIC |DENSITY |MELTING |BOILING | OXIDE | |WEIGHT | | POINT | POINT | --------------------------------------------------------------------------- Magnesium | Mg | 24.36 | 1.75 | 750 deg. | 920 deg. | MgO Zinc | Zn | 65.4 | 7.00 | 420 deg. | 950 deg. | ZnO Cadmium | Cd |112.4 | 8.67 | 320 deg. | 778 deg. | CdO =========================================================================== ~The family.~ In the magnesium family are included the four elements: magnesium, zinc, cadmium, and mercury. Between the first three of these metals there is a close family resemblance, such as has been traced between the members of the two preceding families. Mercury in some respects is more similar to copper and will be studied in connection with that metal. 1. _Properties._ When heated to a high temperature in the air each of these metals combines with oxygen to form an oxide of the general formula MO, in which M represents the metal. Magnesium decomposes boiling water slowly, while zinc and cadmium have but little action on it.