osed to conditions which favor decay, as, for example, railroad ties,
is often soaked in solutions of this salt.

~Insoluble compounds.~ The insoluble compounds of zinc can be prepared by
precipitation. The most important are the sulphide, carbonate, and
hydroxide.

~Zinc sulphide~ (ZnS). This substance occurs as the mineral sphalerite,
and is one of the most valued ores of zinc. Very large deposits occur in
southwestern Missouri. The natural mineral is found in large crystals or
masses, resembling resin in color and luster. When prepared by
precipitation the sulphide is white.

CADMIUM

~The element.~ This element occurs in small quantities in some zinc ores.
In the course of the metallurgy of zinc the cadmium compounds undergo
chemical changes quite similar to those of the zinc compounds, and the
cadmium distills along with the zinc. Being more volatile, it comes over
with the first of the zinc and is prepared from the first portions of
the distillate by special methods of purification. The element very
closely resembles zinc in most respects. Some of its alloys are
characterized by having low melting points.

~Compounds of cadmium.~ Among the compounds of cadmium may be mentioned
the chloride (CdCl_{2}.2H_{2}O), the sulphate (3CdSO_{4}.8H_{2}O), and
the nitrate (Cd(NO_{3})_{2}.4H_{2}O). These are white solids soluble in
water. The sulphide (CdS) is a bright yellow substance which is
insoluble in water and in dilute acids. It is valuable as a pigment in
fine paints.

EXERCISES

1. What properties have the metals of the magnesium family in common
with the alkali metals; with the alkaline-earth metals?

2. Compare the action of the metals of the magnesium group on water with
that of the other metals studied.

3. What metals already studied are prepared by electrolysis?

4. Write the equations representing the reactions between magnesium and
hydrochloric acid; between magnesium and dilute sulphuric acid.

5. What property of magnesium was taken advantage of in the isolation of
argon?

6. With phosphoric acid magnesium forms salts similar to those of
calcium. Write the names and formulas of the corresponding magnesium
salts.

7. How could you distinguish between magnesium chloride and magnesium
sulphate? between Glauber's salts and Epsom salts?

8. What weight of carnallite is necessary in the preparation of 500 g.
of magnesium?

9. Account for the fact that paints made of zinc oxide are not colore