he metal. Magnesium decomposes
boiling water slowly, while zinc and cadmium have but little action on
it.

2. _Compounds._ The members of this group are divalent in nearly all
their compounds, so that the formulas of their salts resemble those of
the alkaline-earth metals. Like the alkaline-earth metals, their
carbonates and phosphates are insoluble in water. Their sulphates,
however, are readily soluble. Unlike both the alkali and alkaline-earth
metals, their hydroxides are nearly insoluble in water. Most of their
compounds dissociate in such a way as to give a simple, colorless,
metallic ion.

MAGNESIUM

~Occurrence.~ Magnesium is a very abundant element in nature, ranking a
little below calcium in this respect. Like calcium, it is a constituent
of many rocks and also occurs in the form of soluble salts.

~Preparation.~ The metal magnesium, like most metals whose oxides are
difficult to reduce with carbon, was formerly prepared by heating the
anhydrous chloride with sodium:

MgCl_{2} + 2Na = 2NaCl + Mg.

It is now made by electrolysis, but instead of using as the electrolyte
the melted anhydrous chloride, which is difficult to obtain, the natural
mineral carnallite is used. This is melted in an iron pot which also
serves as the cathode in the electrolysis. A rod of carbon dipping into
the melted salt serves as the anode. The apparatus is very similar to
the one employed in the preparation of sodium.

~Properties.~ Magnesium is a rather tough silvery-white metal of small
density. Air does not act rapidly upon it, but a thin film of oxide
forms upon its surface, dimming its bright luster. The common acids
dissolve it with the formation of the corresponding salts. It can be
ignited readily and in burning liberates much heat and gives a brilliant
white light. This light is very rich in the rays which affect
photographic plates, and the metal in the form of fine powder is
extensively used in the production of flash lights and for white lights
in pyrotechnic displays.

~Magnesium oxide~ (_magnesia_) (MgO). Magnesium oxide, sometimes called
magnesia or magnesia usta, resembles lime in many respects. It is much
more easily formed than lime and can be made in the same way,--by
igniting the carbonate. It is a white powder, very soft and light, and
is unchanged by heat even at very high temperatures. For this reason it
is used in the manufacture of crucibles, for lining furnaces, and for
other purposes where a r